Some Emacs commands are invoked by just one input event; for example, C-f moves forward one character in the buffer. Other commands take two or more input events to invoke, such as C-x C-f and C-x 4 C-f.
A key sequence, or key for short, is a sequence of one or more input events that is meaningful as a unit. If a key sequence invokes a command, we call it a complete key; for example, C-f, C-x C-f and C-x 4 C-f are all complete keys. If a key sequence isn't long enough to invoke a command, we call it a prefix key; from the preceding example, we see that C-x and C-x 4 are prefix keys. Every key sequence is either a complete key or a prefix key.
A prefix key combines with the following input event to make a longer key sequence. For example, C-x is a prefix key, so typing C-x alone does not invoke a command; instead, Emacs waits for further input (if you pause for longer than a second, it echoes the C-x key to prompt for that input; see Echo Area). C-x combines with the next input event to make a two-event key sequence, which could itself be a prefix key (such as C-x 4), or a complete key (such as C-x C-f). There is no limit to the length of key sequences, but in practice they are seldom longer than three or four input events.
You can't add input events onto a complete key. For example, because C-f is a complete key, the two-event sequence C-f C-k is two key sequences, not one.
By default, the prefix keys in Emacs are C-c, C-h, C-x, C-x <RET>, C-x @, C-x a, C-x n, C-x r, C-x v, C-x 4, C-x 5, C-x 6, <ESC>, M-g, and M-o. (<F1> and <F2> are aliases for C-h and C-x 6.) This list is not cast in stone; if you customize Emacs, you can make new prefix keys. You could even eliminate some of the standard ones, though this is not recommended for most users; for example, if you remove the prefix definition of C-x 4, then C-x 4 C-f becomes an invalid key sequence. See Key Bindings.
Typing the help character (C-h or <F1>) after a prefix key displays a list of the commands starting with that prefix. The sole exception to this rule is <ESC>: <ESC> C-h is equivalent to C-M-h, which does something else entirely. You can, however, use <F1> to display a list of commands starting with <ESC>.