GNUS creates and uses three Emacs buffers, each with its own particular purpose and its own major mode.
The Newsgroup buffer contains a list of newsgroups. This is the first buffer that GNUS displays when it starts up. Normally the list contains only the newsgroups to which you subscribe (which are listed in your `.newsrc' file) and which contain unread articles. Use this buffer to select a specific newsgroup.
The Summary buffer lists the articles in a single newsgroup, including their subjects, their numbers, and who posted them. GNUS creates a Summary buffer for a newsgroup when you select the group in the Newsgroup buffer. Use this buffer to select an article.
The Article buffer displays the text of an article. You rarely need to select this buffer because you can scroll through it while remaining in the Summary buffer.