Basically, there is text as well as images, software, videos, music...anything you want and can imaging. Most ISPs like Comcast do provide users with some newsgroup access but a lot of users don't know this and don't use them. I think Comcast provides something like 2GB per month (from Giganews) but that's it. 2GB should be plenty for text but if you want more than that, the 2GB limit can be met in a matter of a few hours. Some ISPs have dropped access because of under-utilization.
Basically I saw them like email, just that everything was public and could be replied to and you could not delete items. Moderation was also pretty much non-existant, so there were problems with spam, off-topic posts and trolls.