MAC address
The serial number burned into your NIC.
Where it came from
The MAC (Media Access Control) address was defined by the IEEE 802 standards in the 1980s as the hardware identity of a network interface. It is burned into the card at manufacture — your device's permanent name on the local network.
Xerox, later the IEEE, handed out the first 24 bits (the OUI) to vendors: look up "00:1A:2B" and you know it is, say, a specific manufacturer.
How it works
A MAC is 48 bits, written as six groups of two hex digits: 00:1A:2B:3C:4D:5E. The format varies — colons (Unix), dashes (Windows), or dotted cisco (001a.2b3c.4d5e).
The 7th bit (locally administered) flips when software spoofs an address. UWC reformats MACs between styles in your browser without sending data.