Yo, Finnel.

Play-by-post roleplaying is essentially collaborative story-telling. Some of the time all the players are pretty much equal contributors, as far as characters and plot go. Other times there's a game master (or two), who works to guide the story to some kind of ending, while the players describe the actions of characters. You'll learn the ropes fast by joining a game, but there's a bit of roleplay etiquette (which you'll also pick up quickly). For instance, don't mess with another player's characters without their permission. Like, if your character swings a sword, the other player decides whether their character is hurt or not.

Games can have anywhere from 2 to infinite players. In theory.

A good starting place to find games is in Interest Checks, although Roleplayers Wanted can serve, as well.

If you don't like waiting for people to reply, there's RPG Chat, which allows you to play games in real time.

There's quite a bit of variety in how people roleplay exactly -- some people like their characters to have statistics, for example -- but them's the basics. Pick games that look interesting to you, and have fun.

If you have any questions or concerns, there's a suspiciously friendly moderator team that's willing to answer questions. I suppose you can also ask other people, too. They don't need to have fancy green names to be helpful.