Comté is a cow's milk cheese, made in the Jura's mountains, in eastern France. It looks a bit like Gruyère and Emmenthal (without holes), but that's just the appearence, its flavour is much stronger.
You can find more information about it on Wikipédia or on the Comté official website.
If you intend to buy: Because Comté is protected by AOC certification, you can be sure of getting a quality product when you buy it. Furthermore, Comté ages well, and some remarkable cheeses can be found 18 months old, or older.
If you want to use it: Comté is used in many recipes on this site. If unfortunately you're unable to find any, you can use other cheeses of similar type instead, like Gruyère, Emmenthal, etc. (but they will not be as good).