A California roll, in my experience, may be any norimaki which is both a "reverse maki" (i.e. has rice on the outside) and contains avocado and crabmeat. It may well contain other things; I've seen a rather overstuffed futomaki with crab, avocado, egg, and tofu advertised as a California roll.

The interesting thing about the California roll is that it is supposedly the first variety of sushi invented outside of Japan which also subsequently became popular in Japan -- or, at least, in Tokyo.