Gnus
Gnus Startup
As opposed to most normal Emacs packages, Gnus uses a number of different buffers to display information and to receive commands. The three buffers users spend most of their time in are the group buffer, the summary buffer and the article buffer.
The group buffer contains a list of groups. This is the first buffer Gnus displays when it starts up. It normally displays only the groups to which you subscribe and that contain unread articles. Use this buffer to select a specific group.
The summary buffer lists one line for each article in a single group. By default, the author, the subject and the line number are displayed for each article, but this is customizable, like most aspects of Gnus display. The summary buffer is created when you select a group in the group buffer, and is killed when you exit the group. Use this buffer to select an article.
The article buffer displays the article. In normal Gnus usage, you don't select this buffer---all useful article-oriented commands work in the summary buffer. But you can select the article buffer, and execute all Gnus commands from that buffer, if you want to.
Gnus
Gnus Startup