Newsgroups, or forums, are rather like notice boards where you can post and read email. They work as an electronic post box where you can rummage through the messages. They can be open to anyone, limited to subscribers of a particular service, such as CompuServe, or work on an invitation-only basis.
Unlike mailing lists, to see what's been posted to the newsgroup, you need to call in and see what's there. You'll be presented with a list of messages which have be added since your last visit. These may be sorted into topics of conversation for convenience.
This screen-shot gives an example from uk.people.disability in Agent: