Each mech has different attributes and benefits and therefore different purposes. They also have different standard weapon Loadouts. Knowing what your going to be up against is as important as knowing what you're going to be fighting with! Mech's are split into weight ranges and may be available for clan, inner sphere or both. The type of mech (A, J or W) is determined by the -A, -J, or -W added to the end of the name, e.g. Vulture-W. A stands for armor and these mech's have more armor on them, W stands for weapons and means that they carry more weaponry, and J stands for jump jets and allows the mech to use jump jets to jump over obstacles etc...

Light:

Commando - IS
Firestarter - IS
Uller - Clan
Raven - IS
Cougar - Clan

Medium: 

Hollander II - IS
Hunchback - IS
Hunchback IIC - Clan
Centurion - IS

Heavy:

Vulture - Clan
Catapult - IS
Jagermech - IS
Loki - Clan 
Thor - Clan
Mad Cat - Clan

Assault:

Awesome - IS
Atlas - IS
Mascari - Clan

Links:

Official Site - (http://www.mechcommander.com/info/mechs.html) - Contains a list of new Mech's included MCG - Currently Unavailable

Strike Base Alpha - (http://web.utk.edu/~tone/mcsba/) - Contains details of Mech Commander mech's & components.