"Key" seems to mean quite a few different things. Good catch, it would involve a lot of manual retagging though doesn't matter anymore though :P
Let's see:
Keyboard keys
Both the physical ones on the keyboard and the conceptual ones like hotkeys, key-combos, and keystrokes that perform functions. Mostly in combination with keyboard, keyboard-shortcut, keyboard-layout, hotkeys.
Registry key
The ones used in Windows Registry. Mostly in combination with registry.
Cryptographic keys
Encryption keys as in PGP. Mostly in combination with php or cryptography.
SSH keys generated by ssh-keygen tool. Mostly in combination with ssh.
Database keys
Keys as used in databases. Mostly in combination with odbc, access, etc.
Serial numbers
CD keys, product keys, serial numbers, et. la. Mostly in combination with registration or activation.
