MAC is an acronym for (I think) Machine Access Code. It is a unique hardware address for the ethernet controller, which is allocated by the manufacturer and cannot be changed.

When you configure the system you can presumably allocate an IP address. Programs within the TCP/IP suite will then be used to determine the MAC address of ethernet controllers on your network for each IP address. When an IP message is sent onto the network, the message will contain a destination MAC address.

Actually it is the Media Access Control address.
This is a unique address given to hardware devices, but on some devices, this is programmable, (although I would not recommend messing with this).
The other info tomatge gave is basically correct, but if you would like a deeper understanding of ethernet, and TCP/IP, I would suggest www.iol.unh.edu/training/ethernet.html, and www.pugettech.com/net101/. These are some good tutorials on networking.

Best of luck.

The MAC address can usually be found on a sticker or etched into the controller card itself. The MAC will look similar to this: ab-00-11-22-33-cd not always hyphen seperated but almost always in hexidecimal form and 12 characters. (If you run into a binary MAC it will be 48 bits). Sometimes the MAC is also refered to as the Hardware or Ethernet address. All ethernet comms will have a layer 2 address (MAC) and can have a layer 3 such as IP. You will never have IP without MAC as the IP address is dependant on the MAC for physically locating the devices.

Go to your favorite search engine and type in MAC address or TCP/IP or OSI Model for more information. Have fun!