On a forum, you have lots of members talking to each other.
On a Fediverse server, you have lots of people mostly talking to people on other servers.
Not all servers are themed, for example mas.to, mastodon.social, mastodon.online, mstdn.social are just general servers.
The ones that are themed may do so for lots of different reasons such as agreed rules, shared identity (shown in the address ending), funding (if you're a member of a co-operative) etc.