A forum is a format for discussions (such as opinions) and Q&A (such as how-to) questions.
All of a forum's content is crowdsourced. This is different from a blog, whose material is authored by specified authors; from a wiki in that a wiki page's author is a permissioned person in the community and in a gallery, whose content is furnished by gallery owners. A gallery has nearly unlimited media space, limited only by the setup of the community. Both forums and galleries have RSS.
A Telligent forum has 2 features that aren't in any other application:
Please follow the instructions on the What is an application? article.