Then there's everyone's favorite - The Troll. These are people (and I employ the term here quite loosely) who use various techniques to draw others into arguments or pointless debates.
The term comes from the fishing technique by the same name, which involves pulling bait or a lure on a line behind a boat, hoping the fish won't see that it's a trap and will hook themselves.
Trolls can sometimes be subtle, and seem like the usual confused person expressing an opinion. They can sometimes be obvious, such as the ones asking for advice and refusing to acknowledge or consider what's offered. And they can sometimes just come out with it, posting outrageous nonsense that's guaranteed to start an argument if anyone takes them seriously.
If you suspect you're dealing with a troll, either flag their posts or, if there's a senior person involved who's engaging them after stating what they are, pop some popcorn and enjoy the show. A few of the more experienced people here will often carry on the discussion in order to let the less experienced become more familiar with the mindset of trolls.
Trolls are sneaky, cowardly creatures. They like to start trouble and then sit back and watch as others argue. If you call them on their games, they may respond by stating some seemingly harmless reason for their comments. They most commonly throw a tantrum or start to devolve into even lower life forms, right before your eyes.
Some will switch gears, and claim to be engaged in "performance art." Those people should be laughed out of the place, as they deserve.
Unless you're really experienced in dealing with them, your best bet is to flag their posts, and explain it with the phrase, "Looks like a troll." The mods can usually recognize the species pretty well. If they can't tell for sure, it's probably not a thread that needs to be left anyway.
Don't confuse trolling with outright racism or other hate speech. There's a name for animals who post that sort of thing on public forums, but it's not suitable for use here. The best use for such creatures is hauling large piles of heavy rocks from here to there and back to here again - ad nauseum.