I guess its a thread that stays up top? Or near the top. Depends on how many "stickies" there are.
Do the moderators decide this? Generally, it's the mod or mods for that particular forum.
What is the deciding factor to make it one? Mostly it's a judgement call. It could be a topic that keeps coming up, over and over. It could be a topic that is important to that forum. It may be specific rules for that particular forum (the title of the thread is usually meaningful - if it's rules, it will say so).