Vanilla's user-generated content is organized into Discussions. Replies or follow-ups to those Discussions are called Comments. Each Discussion can have many Comments, or they could have none.
Here's a Discussion with just one Comment:
Some communities or forums will refer to Discussions differently --- some folks may call them posts, threads or topics. Like most verbiage strings, these can be changed by contacting your CSM, but the underlying functionality remains the same.
Vanilla supports several different Discussion types:
Each of these will create a brand new Discussion, that other users can then comment on (if the Discussion type is "Question", comments on that Discussion are called "Answers").
While every community will have access to Discussions & Comments, other discussion types can vary by plan level. If you are unsure, consult your Account Executive or CSM.
More info on various discussion types: