@j95 covered them all in his awesome infograph (is that what they're called?) It's awesome by the way. Feeling insecure, taking out their problems on someone else and trying to fit in. I like what @safari93 said about Mean Girls, about there being a hierarchy so that bullies who end up at the top, get away with bullying anyone below them and because they're at the top, there's nothing people, especially students, although some teachers might try if they were told, do about it.
Bullying does just happen in schools, where else does it happen? What can be done to prevent bullying in these places? Bullying happens everywhere. At home or at work are the main perpetrators, and of course online. But it really does happen anywhere, out on the street, clubs (as in yoga groups or sport teams. But also dancing/drinking clubs) or other meeting places. Prevention in homes. If the parents aren't involved, they should look toward stopping the bully, and helping the victim. If they are, then depending on the situation going to a teacher or other third party adult, a friend or friend's parent for example, to help the parent or help stop them. Online is difficult, very difficult to control because the online world is so large. In forums, the moderators would be in charge of dealing with cases of bullying, so turning to them would help you. The biggest thing with online bullying is to ignore, especially the trolls, if the bullying is just some online person teasing you, that you don't know them. The best thing to do is ignore and block. If you do know them, taking outside measures such as parents or teachers if applicable to help with the situation is a yay. I've lost track of thought....
Prevent bullying by ignoring and telling! (and online, blocking) so that the bully doesn't get a reaction(ignoring), can be reprimanded by authorial figures(telling), and can't talk to you or you won't even see their messages(blocking).
What would you recommend to somebody being bullied? I would tell them to tell someone. It might seem like your a 'tattle-tale' but that is a false social construct to stop people from informing adults about bullying or other events that should not of happened. (I could go on for a while on how much 'tattle-tale' bothers me). I would also tell them to ignore the bully. I know personally that it can be very, VERY difficult to ignore and the feeling to rise to the bullying can be strong, but ignoring is one way to go, especially in a school environment where you might not have to interact with the person, even if they are in your class, you might not have to do a task or something with them.