Media Access Control Address
Is a hardware
address that uniquely identifies each node of a network with a 12 digit
reference. Each hardware manufacture that makes network equipment is given
a unique reference (the starting part of the MAC address) to use and to
apply to each piece of network equipment they make.
The MAC layer (part
of the OSI 7 layer model, yeah yeah enough already) interfaces directly
with the network medium, thus each different type of network equipment
must have a unique identifier, else there'll be chaos.
A simple example of the chaos would be to liken it to shouting out the
name 'John' to a room full of people called John, they would all reply and
confusion would reign, ok ok that was crap but you get the idea.