According to Deann and Rick Bayless in their book Authentic Mexican : Regional Cooking, the metate was "The last of the techniques that set Mexican cooking apart is the grinding. It began millennia ago with corn crushed on a rock slab (metate) to make the dough for tortillas. The slab proved good for grinding rehydrated. chills, for nuts and seeds, for cacao beans and even the tender curds of fresh cheese".
Mayan-style metate y mano - Picture by GourmetSleuth.com
Aztec-style Picture by GourmetSleuth.com
The traditional Aztec metate was a slanted slab with three short legs, similar to the one pictured below. This is the style of metate frequently found in Mexico. Frequently the surface is more slanted than the one shown here. The slant allows the user to work the ingredients downward into a bowl at the end of the metate.
The Mayan version sits flat on the ground with a slightly indented top grinding surface (like the one shown at the top of this page).
Costa Rican Metate Photographer: Unknown
This metate is similar to the Aztec version with three legs but quite a bit taller. The user would probably sit on a stool, rather than on the ground when grinding with this tool.