Rice vinegar is just what it sounds like - vinegar made from fermented rice. Be careful not to buy seasoned rice vinegar, which can contain a lot of things you don't want, like salt and corn syrup.
Where do I find rice vinegar?
Rice vinegar can be tricky to find, depending on what your grocery store is like. If it's not with the rest of the vinegars, check the international section, or just pick it up at an Asian market.