Roman Catholic specifically refers to Catholics of the Latin Rite (the largest).
There are Byzantine Catholics, Melkite Catholics, etc. Overall there are 23 rites. The Latin Rite is the Western Church and the other 22 are Eastern Rites.
http://www.ewtn.com/expert/answers/cath … urches.htm gives a good explanation.
The Universal, Catholic Church is one, unified body, but the faith developed slightly different in each rite. Most of the differences are liturgical logistics, but there are others as well.