What is the difference between traditional Mexican cooking and Tex-Mex?
I'm not an expert by any means, but here's my understanding of the situation:


There are many different regional cooking styles in Mexico, not all similar to the northern Mexican food upon which Tex-Mex is based.
Overall, relative to Tex-Mex cooking, there tends to be less focus on heat, cheese, and beef.
Cheddar cheese is a more recent addition in some northern Mexican cuisines (eg, Sonoran).
Although wheat is used (for rolls, empenadas, etc.), flour tortillas are typically only in northern Mexican cuisine.


Common "Mexican" dishes that were created in the United States : fajitas (Tex-Mex, inspired by arracheras), mission-style burritos (San Francisco), chimichangas (Arizona).