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.