Each account has a 'world' icon next to it, and corresponds to a news server on the Internet. Expand the tree against a News Account to show the groups you're subscribed to on that server. Accounts are shown in bold if their subscribed groups have any unread messages. The number of unread messages in each group is shown in brackets next to the group name.
The Subscribed Groups tree has its own right-click popup menu with options to get messages, subscribed to new groups and accounts, etc.