In my view point forum posting is similar to question answering , just sharing the suggestions, that is concept of forum and question and answering.
question answer and forum posting sites both are same, I mean both are using for sharing the information
From SEO perspective there is a difference, most of the answering sites are nofollow, very few answering sites allow you to hyperlink the keywords. So if you are looking for good rankings and generating back links for your site forums are a better option.
I think in forum posting we do so many task like share some think, get help from outside, get suggestion from others. where as in question ans we post only question and get ans from outside, or we give ans of posted question.