This is my first Instructable! Please give advice, constructive criticism, AND PRAISE! (I can't believe I forgot to list the last one. Jeez!!! :)
[EDIT: As one commenter suggested, (and I failed to realize) your mat should have a conductive top layer and a non-conductive bottom layer, NOT the three layer /resistive-conductive-resistive/ version I have made here. I haven't yet found a proper substitute for the aluminum (it is not durable enough to use as a top layer). I will edit this -Ible to reflect the correct 'two-layer' assembly once I have the proper material for an acceptable price! (Free.... Obviously...) Thanks for reading on!]
Step 1: Explanations
To be more accurate, an Anti Static mat drastically slows the rate of static energy discharge or maintain at zero, the difference in the common electrical potential of the operator relative to the many components he, or she, may come in contact with. This eliminates the possibility of discharging the static into a sensitive electronic component possibly causing it irreparably damage.
The mats are typically constructed of an electrically resistive rubber or plastic derived material combined with a conductive layer that leads to a ground plane such as a three pronged outlet (here in the US) or a grounding rod in the ground outside of your hobby shop. See the two images above for an example.
Some mats implement a direct connection with the operator through a wrist strap or floor mat. The bench top mat that I am constructing will use a wrist strap to provide a complete circuit from the skin, through the mat, and into the chosen grounding plane. As I understand things, this setup can provide a more stable and consistent rate of discharge over a day's work period.
ESD mats will usually have at the least a 1 mega ohm resistor inline with the circuit somewhere to slow the rate of ElectroStatic Discharge and prevent shock from the grounding plane reaching the operator.