You create new or respond to existing topics here that are opened for discussions, sometimes friendly debates. It's easy to browse around in different topics, the discussions stay there visible, people can always add to it until the topic is closed.
The major differences between Forums vs Main Feed and Blogging:
It usually calls for a discussion, a debate that members can participate to for a long time, responding to each other publicly back and forth, share their opinions, doing basically the same thing they would on the main feed, but it lasts longer and visible longer, newer members can join to older discussions easier too.
Please note, content posted in the wrong category or section may result removal.