A key sequence (key, for short) is a sequence of input events that are meaningful as a unit--as "a single command." Some Emacs command sequences are just one character or one event; for example, just C-f is enough to move forward one character. But Emacs also has commands that take two or more events to invoke.
If a sequence of events is enough to invoke a command, it is a complete key. Examples of complete keys include C-a, X, RET, NEXT (a function key), DOWN (an arrow key), C-x C-f and C-x 4 C-f. If it isn't long enough to be complete, we call it a prefix key. The above examples show that C-x and C-x 4 are prefix keys. Every key sequence is either a complete key or a prefix key.
Most single characters constitute complete keys in the standard Emacs command bindings. A few of them are prefix keys. A prefix key combines with the following input event to make a longer key sequence, which may itself be complete or a prefix. For example, C-x is a prefix key, so C-x and the next input event combine to make a two-character key sequence. Most of these key sequences are complete keys, including C-x C-f and C-x b. A few, such as C-x 4 and C-x r, are themselves prefix keys that lead to three-character key sequences. There's no limit to the length of a key sequence, but in practice people rarely use sequences longer than four events.
By contrast, you can't add more events onto a complete key. For example, the two-character sequence C-f C-k is not a key, because the C-f is a complete key in itself. It's impossible to give C-f C-k an independent meaning as a command. C-f C-k is two key sequences, not one.
All told, the prefix keys in Emacs are C-c, C-h, C-x, C-x C-a, C-x n, C-x r, C-x v, C-x 4, C-x 5, C-x 6, and ESC. But this is not cast in concrete; it is just a matter of Emacs's standard key bindings. If you customize Emacs, you can make new prefix keys, or eliminate these. See section Customizing Key Bindings.
If you do make or eliminate prefix keys, that changes the set of possible key sequences. For example, if you redefine C-f as a prefix, C-f C-k automatically becomes a key (complete, unless you define it too as a prefix). Conversely, if you remove the prefix definition of C-x 4, then C-x 4 f (or C-x 4 anything) is no longer a key.
Typing the help character (C-h or F1) after a prefix character displays a list of the commands starting with that prefix. There are a few prefix characters for which C-h does not work--for historical reasons, they have other meanings for C-h which are not easy to change. But F1 should work for all prefix characters.