Follow-up:

> One posts individual messages (like one email) to a group

Messages can be posted to more than one group; this is called "cross-posting", much like with email mailing lists.

> The end result, with a good news reader program, is somewhat like a large global collection of topic-specific forums.

You can also choose to "subscribe" to a subset of the available groups; your news program is able to pull metadata only for that subset you care about. You can also download individual messages on demand, much like IMAP email access. It's fairly bandwidth efficient for clients.