Documentation for a Key
The most basic C-h options are C-h c (describe-key-briefly) and C-h k (describe-key). C-h c key displays in the echo area the name of the command that key is bound to. For example, C-h c C-f displays forward-char. Since command names are chosen to describe what the commands do, this is a good way to get a very brief description of what key does.
C-h k key is similar but gives more information: it displays the documentation string of the command as well as its name. This is too big for the echo area, so a window is used for the display.
C-h c and C-h k work for any sort of key sequences, including function keys and mouse events.
