A forum is a website where users can post comments to discussions about a topic.
A Forum of Ice and Fire is a place on the web where fans of Game of Thrones can discuss anything and everything about the popular book and television series. Discussions are split into distinct topics, such as 'Season 6', 'the Dothraki', 'Dragons' etc. All users have to do to take part is sign up and post a comment.
Forums are usually moderated by an administrator who checks comments that other users flag as inappropriate and has the power to ban users who become offensive. Offensive users are known as trolls.
Forums and comment sections on a website are a fantastic way to add user generated content to your site. Content posted in forums is relevant, frequently updated and can be crawled and indexed. If you go for a forum be careful of forum spam