Slayer

Has nothing to do with the anime Slayers.

Slayer is a title that gets bandied about a lot on /tg/, but the two most prominent /tg/ references are as follows.

Warhammer

In Warhammer Fantasy, dwarves who feel too ashamed to live with themselves take up a ceremonial pledge to the god Grimnir. They tattoo themselves, dye their hair red and spike it into a huge mohawk, grab the biggest weapons they can, and set off to redeem themselves through dying in battle. Of course, they can't just kill themselves; they have to fight as hard as they can and to the very best of their abilities, until they fall against a nasty legitimately too mighty for them to overcome. Sort of like a super-metal paladin, or a non-weeaboo dishonored samurai.

The most famous Slayer is the titular star of Gotrek & Felix.

In Warhammer Fantasy Roleplay, Slayer is a career path that dwarves can enter, but it is a one-way trip; other than the more powerful levels of Slayer, they cannot ever change out of that career. It's a brutal combat machine, but utterly useless at anything else.

Pathfinder

In Pathfinder, the Advanced Class Guide splatbook introduced a bunch of new classes, which basically ripped off the idea behind 4th Edition Dungeons and Dragons's Hybrid Classes: each class was basically a fusion of two existing classes. The Slayer is a Rogue/Ranger hybrid, essentially a beefier rogue with the ability to select a mixture of rogue and ranger talents and the ranger's favored enemy trait matched to a stronger version of the rogue's assassination traits (Sneak Attack, Studied Target). Great if you really, really want to play a Bounty Hunter type character.

Ignore a lot of what that guy said. Slayers are what happens when you take the the combat abilities of the rogue and ranger, but ignore the rest of their abilities. They are good at tracking and stealth. They have a weaker version of favored enemy that they can apply to any one target at a time as a move action. They get sneak attack dice every third level instead of every odd level.