Emacs buffers have modes associated with them. The reason for this is that your needs when writing a mail message are very different from your needs when, say, writing a program. Rather than try to come up with an editor that would meet every single need all the time (which would be impossible), the designer of Emacs chose to have Emacs behave differently depending on what you are doing in each individual buffer. Thus, buffers have modes, each one designed for some specific activity. The main features that distinguish one mode from another are the keybindings, but there can be other differences as well.
The most basic mode is fundamental mode, which doesn't really have any special commands at all. In fact, here's what Emacs has to say about Fundamental Mode:
Major mode not specialized for anything in particular.
Other major modes are defined by comparison with this one.
I got that information like this: I typed C-x b, which is switch-to-buffer, and entered ``foo'' when it prompted me for a buffer name to switch to. Since there was previously no buffer named ``foo'', Emacs created one and switched me to it. It was in fundamental-mode by default, but it it hadn't been, I could have typed ``M-x fundamental-mode'' to make it so. All mode names have a command called <modename>-mode which puts the current buffer into that mode. Then, to find out more information about that major mode, I typed C-h m, which gets you help on the current major mode of the buffer you're in.
There's a slightly more useful mode called text-mode, which has the special commands M-S, for center-paragraph, and M-s, which invokes center-line. M-S, by the way, means exactly what you think it does: hold down both the and the key, and press ``S''.
Don't just take my word for this--go make a new buffer, put it into text-mode, and type C-h m. You may not understand everything Emacs tells you when you do that, but you should be able to get some useful information out of it.
Here is an introduction to some of the more commonly used modes. If you use them, make sure that you type C-h m sometime in each one, to find out more about each mode.