It is any currency pair other than the US dollar that is considered a cross, and it holds no less profit potential than the majors. When you have too much exposure to the US dollar, all your trades will tend to run in the same direction, which can be problematic if that direction is against you.
There is less trading of "exotic" currency pairs, as well as a much higher cost of buying or selling them. Exotics are not as expensive as you might think, and many of the greatest traders of all time have made their fortunes trading them.