Daedalus,
There are a number of ways of securing a wifi network.
One is to only allow connections from network cards with a MAC (Media Access Control) address (the 12 digit number) that is on a pre-approved list.
In addition to this, access to the network may require the provision of a "key" - this is what WEP and WPA require.
The key can be provided either as the 64, 128 or 256 bit string, or more normally as a passphrase that generates the key when hashed.
Another way to prevent access to a WiFi network is to turn off the network ID broadcast (SSID). In this case you have to tell your network device what the SSID is before you can connect.
It is further possible to limit the number of IP addresses given out by the DHCP server on the WiFi network - when used with address reservation this can prevent you from using the network even if all three above are in place.
All the above is at the MAC or IP layers - layer 2 & 3 controls.
Then there are networks to which access is controlled by an authentication service - application level control - such as BT Openzone or similar commercial wifi "hotspots". Here you need to register and pay in some way for the service.
It is possible to operate this kind of authentication requirement WITHOUT requiring payment I should add - in which case you still need to have some form of account ID and possibly password given to you.
HTH
SD