Acronym for Media Access Control.
This is the LinkLayer?, and is usually talked about in Ethernet where it has a very simple structure:
| from hardware address | to hardware address | protocol field | <data>
The term MAC Address is also used to refer to the hardware address of an Ethernet card. These are usually hard-coded in an Address ROM on the network interface card.
A MAC address is 48 bits long, and is commonly represented as a colon-delimited sequence of 12 hex digits:"ab:cd:ef:gh:ij:kl".
The low nybble of the first byte (the "b") contains a lot of information, the lowest bit of this means that the traffic is multiple destination (ie, multicast or broadcast). This is because on the wire each byte is sent low bit first.
The first 24 bits/6 digits (the "ab:cd:ef") identify the manufacturer of the network card, as overseen by the IEEE and IANA. See 
http://standards.ieee.org/regauth/oui/oui.txt for more information.
A MAC address should be globally unique - no two network cards should ever have the same MAC address. Note that some network cards (notably, those used with Solaris machines) may be software-reprogrammable.
Recognising various mac addresses:
Some common mac addresses you'll see (And get confused about):
|1:0:c:cc:cc:cc|Cisco discovery Protocol (CDP)
|1:0:c:0:0:0|Spanning Tree Protocol (STP)
|1:0:c:0:0:0|Cisco Interswitch Link
|1:0:5e:x:x:x|IP Multicast (
RFC:1112)
