I like to go to forums to see all the differing sentiments on a subject; sometimes opinions from many people can help you make a decision if you have a certain problem that the topic solves, for instance. Other times, it's fun and grants you a good feeling to share your opinion and your voice, although nowadays most forums are "cliqued" up, meaning that only a certain type of person will join them and share one kind of opinion, removing some facets of the creativity in responses and opinions you would get in, say, the real world. I only join forums where this is not the case, though, like this forum.
The major part of forums, though, is the sharing of information through text. That's all. It's no different from discussing things on a social network or talking peer to peer through your phone or through instant messaging, save for the fact that posts are more thought out and organized, don't you think? There are many gems in terms of information on this forum, if you really need an example.