I've never involved in any roleplay before and I'm very curious how it works here?

Isn't roleplay mean we are carrying a character and then interact with other characters? but how do we do that here? 

I stumbled upon a roleplay before in a social website and it was done in a forum. So they just basically write a few sentences each person (only controlling their own characters) each post. But we can't do it like that here, right?

So how does it work here?

and can we let our character interact with many characters or we just had to stick to one storyline with one partner?

....

everything about roleplay is very confusing to me.

anyone can explain?