Spears skill determines your effectiveness while using polearms. Spears are generally much heavier than standard weapons, and are usually quite slow, but have high damage output to offput it. Spears most commonly do stabbing damage, though some do Slash as well.
High-end spears such as the Chainsaw lance, Garden Weasel or Zap Brannigan tend to be very damaging and scale very well with brawn and endurance. They also can accept a wide range of mods compared to other weapons. Every spear is also at least PC1, and still more are PC2, allowing you to parry fists and whips with ease. The high endurance also helps greatly with drugs, so most spearmen are expected to get high as a kite on coke and speed. Spears also have a +5 bonus to parrying on all two-handed spears, and a 2.5 bonus on all one-handed spears.
Spears are very heavy, compared to other weapons. They're also often quite slow and take a lot of reflexes to reach good speed, with many spears requiring around 31 reflexes to hit top speed. Even then, they're still out-sped easily by Whips and Fists. Also suffers from lower to-hit compared to other endgame weapons, even with drugs, unless you take Chromemouth and max it out.
Here's a quick overview of some of the more notable spears in the game. A more exhaustive list can be found on the Weaponry page.
Military Fork - The first spear for newbies who pick the spears class without any XP under their belts. Nothing amazing, but it'll punch through chuds just fine until you get the cash and experience to get an upgrade. There are plenty of other spears in this same category of "not great, but it'll do", but the military fork is extremely easy to acquire and has generally well-balanced performance, so it's rarely a bad pick.
Infected Syringe - Infects anyone who it hits without Hyperimmune with Ebola and causes large amounts of bleeding. Sometimes used by serial killers and drops off of Doctor Shunt. This isn't a one-shot machine, however; Ebola is only truly dangerous to newer players, who haven't maxed out their HP pools yet and don't have the medic skill to cure the ailment.
Assegai/Luftspeer - These things are footnotes. Used in crafting the Large Scythe, but otherwise inferior to the Abossegai in every way. Savage Michael gives an assegai upon killing him once per roll, and they drop from sky pirates in the Necropolis Gate. Luftspeer is a once-per-roll reward for killing a boss in the Dos Kochenhammer. It's just an assegai that comes fully modded with a shock generator that does an additional 1-5 damage over the norm, but it's still an assegai, and thus worse than the abossegai.
Large Scythe - Crafted. Can reach a respectable 3.0 speed without too much trouble (especially with a balanced grip) and gets fair to-hit. Can comfortably reach the low 50s and even approach 55 in slash damage if you've got the brawn for it, along with a bit of free stab damage. Outclassed by the Chainsaw Lance at the top end, but much easier to max out than the lance, and still does a very creditable job while having better to-hit.
Pitchfork - Dropped by farmers in Macero, as well as the odd freak in the Toxic Dump. Has about the same impressive damage profile and to-hit as the Large Scythe, except with stab instead of slash (slash is slightly better) and no secondary damage. The main difference is that the pitchfork is about .5 slower, and lags behind even with a balanced grip. On the other hand, it has an excellent crit rate that can reach well over 50% without really trying. Ultimately inferior to the Large Scythe unless you're willing to properly buff up or lean on crits, and in either of those cases, you're likely better off just using a Chainsaw Lance. Not a terrible weapon, but it's generally outclassed by its peers.
Corrosive Pike - Does rare acid damage that can scale high with enough brawn, and gets respectable to-hit. Followed by an undeserved reputation of being "average", this thing is a diamond in the rough. Can only accept two modifications; one of these should absolutely be the monowire serrator, to max out the weapon's acid damage potential. Not much good in PvE, because few mobs are weak to acid that aren't weak to everything else anyway and the pike's a one-trick pony outside of its special typing.
Abossegai - Other weapons wish they were an Abossegai; this thing's a monster. High minimum damage, and the max can hit stab damage in the high 40s (scaling with brawn). Also one-handed, leaving your other hand free for either grabbing/nanite spam/other mischief or horrendously powerful dual-wielding. To top it all off, relatively easily acquired in the Necropolis.
Willy Pete - Crafted. Not terrible, but a somewhat dubious pick in most cases. One handed like the abossegai, which is nice for DPS and crowd control. Max damage caps a little over 50 (scaling very well with brawn), but minimum damage is a paltry 3, and all of that damage is burn damage. Few mobs are weak to burn aside from nullianac packs, and even when fighting them, the abossegai enjoys a consistency that the willy pete never will. Also gets one point to-hit less than the abossegai, although it is also .1 speed faster.
Garden Weasel - The straight-laced traditionalist's spear. No fancy tricks or special damage types, but gets good to-hit and good speed without much difficulty, is very lightweight, and can hit slash damage in the mid-40s easily enough (pushing 50 with a bit more effort), while also having a fair minimum damage of 9. Found either from toxic freaks or from the chud-only shop.
Chainsaw Lance - A rather obnoxious weapon that will only tolerate strictly excellent stats in its wielder, or else it will sulkily put out only mediocre performance. Difficult to speedcap at 31 reflexes, but caps at a nippy 2.6 speed. Also requires top-notch brawn to exploit its high damage output, but can reach an incredible 55+ slash damage when properly babysat (and can even push 60 with a bit of doing). That's on top of 4-8 bleed damage and an excellent crit rate. Suffers from an innate to-hit penalty and cannot accept any grip mod aside from the grip tape mods. Monowire is a must for this to max out its damage.
Titanium Bo Staff - Crafted, with the schematic being rather difficult to acquire and the recipe itself quite tedious. However, staff itself is fast and very accurate by spears standards. On the one hand, the damage profile isn't immensely impressive, capping out around the high 30s or low 40s, but on the other hand, that's beat damage, which is a very scary damage type that can dish out broken bones and knockouts quite easily. Cannot accept a monowire serrator (or, obviously, a sharpening kit).
Zap Brannigan - The former best spear in the game. Fairly difficult to craft, but gets excellent to-hit and does a fair amount of stab damage, capping out somewhere in the 40s. Assuming you remembered to turn it on, it'll also put out 6-16 electric damage. This is particularly nasty in PvP, where few people soak electric damage well. Does not accept the monowire serrator or elemental mods, so all you can put on it is a sharpening kit and a grip.
Here's a few ideas on the best mods to rock on spears, and when they're most useful.
Elemental - It's best to add mods that contribute to the weapon's base damage, so butane kits are a good idea for the Willy Pete. The Zap Brannigan, sadly, cannot accept a shock generator. The acid pump kit is a solid choice when applied to the Corrosive Pike's damage, but beware that with this and the serrator you will have no more room for a grip mod, which will leave the job of getting the weapon to a good speed and accurate to-hit to your stats.
Grip - In PvE, you'll generally want to use the balanced grip, to counteract slow speed on the majority of spears. In PvP, drugs should be used to counter-act the slow speed, and you should use a gyro grip instead. The exceptions to this are the Corrosive Pike, where you may want to drop a grip mod for two other mods, and the chainsaw lance, which cannot accept any grip mod except for grip tape.
Monowire Serrator - A solid choice on any spear that will accept it; brawn scaling is the key to most spears' high damage, although neither the Zap Brannigan nor the Titanium Bo Staff will accept a monowire. If you aren't brawny, of course, then you can leave this out.
Sharpening kit - This is a straight buff, so go ahead and stick it on any spear that will take it. The exception is the Corrosive Pike, on which it's a total waste of a precious mod slot.
What to consider when picking your racial mutation:
Abomination - A popular combination back when abomination itself was popular. Hideous Freak and abomination both buff endurance and the latter in particular also boosts brawn. The synergy with abomination and drugs also helps tons since spears rely on drugs to overpower their opponents.
Hideous Freak - Should you choose to forego brawn and pick senses instead as your third stat, Hideous Freak by itself should prove to be a worthwhile racial, taking your focus to the levels of endurance whips, at more than 40 without buffs.
Vampire - The buff to brawn takes the brawn-scaling spears to a whole new level, but since vampire is incompatible with the two most popular endurance buffing mutations, there's a net zero gain in endurance. Also, being a vampire sucks.
Zombie - Buffs endurance and brawn, helping you take advantage of brawn-scaling weapons easier while also giving you more of the much-needed to-hit. Zombie also gives some resistance to drug addiction, an invaluable addition to a spearman's arsenal. Zombie's natural slowness means that the slow speed of some spears such as the large scythe or the chainsaw lance, aren't as crippling. On top of all that, the spears' tendency towards tanking out your opponents because of their high parry bonus go hand in hand with zombie's natural tankiness (the soaks and the damage reduction).
Chud - You can access garden weasels much easier than most by simply buying them from the chud-only shop, rather than waiting for another chud to buy it for you or finding it in the toxic dump.
Chromemouth - Chromemouth buffs endurance by 3, a significant buff without many downsides, making it the natural choice if you've been turned off by every other racial. You'll have a sizeable to-hit and some minor bonuses to useful skills. However, you won't really be able to take advantage of Chromemouth's main draw, Zero, because you won't have the cool to pull it off.
PvE Assuming that you're brawny, the abossegai is so incredibly strong that it can punch through most mobs no problem. Dual wielding is a source of fantastic DPS, but single wielding gives you the option of using grab as a source of crowd control. This is an integral part of soloing ants and basilisks, in particular. Other weapons are situationally useful (the bo staff is a great junker buster, for instance), but you can't go too far wrong with an abossegai.
PvP PC2 on the vast majority of spears is a definite bonus in PvP, and the chainsaw lance can cut through most soaks by value of bleed damage. However, its low accuracy can be a turn-off if you're fighting particularly dodgy players. Dual abossegais are brutally strong if you are the brawny type. Garden weasels, bo staves and zap brannigans are accurate, fast and deadly, the top choice in most situations.