I believe they do the same thing under the covers. RSpec originally used the "context/specify" pair, and somewhere on the road to 1.0 decided that "describe/it" was a nicer DSL, but context/specify is still supported. I also seem to recall that some prefer context/specify to, er, describe certain instances, but I can't remember which or why. Purely a readability/semantic though, though, not a functional difference.
Thanks, so "describe" and "it" are just alias
Actually, "context" and "specify" are the aliases of "describe" and "it".