Gluten free diet means a diet that is completely free of ingredients derived from gluten containing foods. Some of these gluten ingredients include wheat, barley, rye, oats and triticale. Several grains and starch sources are acceptable for a gluten-free diet.
JuliusWAI had a very good answer! Just one thing to add though, a lot of the people on here are saying that you can't have breads and stuff but that's not true. Many of the same foods are made with rice flour as an alternative. Some of them are actually better in my opinion :) Here's a link as to what exactly gluten is: http://en.wikipedia.org/wiki/Gluten-free_diet
I am actually on a Gluten free/Wheat free diet. they are basically the same, and when it is in one food, they are both in that food. Just stay away from Breads, Doughs, Cakes, Cookies, Flour, noodles, pastas, etc. You can usually buy gluten free items at a Henrys or a Sprouts in the gluten free isle. I hope i helped/ answered the question right ;)