A typical and minimalistic structure for a discussion board is:
Forum -> Thread -> Post
Then, you can group forums in categories and forums may contain subforums.
But, it greatly depends on the workflow of the system. In order to get a feeling of the terminology used, it'd be better to have a look at the history of such systems (e.g. Bulletin Board systems) and the concept of ancient agoras/forums.
But, in order not to start over-describing the whole concept, it's all about practicality and distinguishing online and offline discussions/concepts. So, offline, you have discussions on topics, and online you have conversation threads containing posts (like posts in a real bulletin board).
A comment provides additional information/input on something else, whereas a post provides a complete answer.
Imagine someone asking if "Twitter's 'tweets' are different from Facebook's 'posts' and StackExhange's 'answers'". It wouldn't really be relevant to ask if 'tweet' is a valid word, right? It's their system, they may call their 'units of dialog' whatever they want ;). I could tell someone that I "read their comment on tweeter", and they would have every right to look at me strange but they'd nevertheless be able to understand what I meant.