Tofu basics:

There are several types of tofu and its based on smoothness/firmness

silken -- as name implies it is very smooth like a custard, used in miso soups, yu-dofu, hiya-yakko in Japan and in Mapo-Dofu or Homestyle-tofu chinese.

firm -- not as smooth nor as soft. it can hold its shape while cooking

extra firm -- as the name implies it is very firm and will hold shape while cooking as in stir frys. The texture tends to be coarser more like cottage chesse. Extra firm or firm is used in things like Yaki-dofu (grilled tofu with teriyaki like sauce).

Depending on the supplier, the firmness can be rated with different names and catagories.

One quick way to tell is look at the smoothness of the surface. If its very smooth and has a sheen or mirror like surface its silken. If it has a pattern like a woven fabric (because its pressed with fabric to remove the water) then its firm or extra firm.

Which one you want to use is your personal preference with texture. For recipe's check Vegetarian cook books or Vegan cookbooks such as:

Laurel's Kitchen (highly recommended for anyone wanting to go vegetarian)
Moosewood Cookbooks ...