Quick rundown from what I know being involved in Florida:
Each county has a Republican Executive Committee that is made up of elected or appointed Precinct Committeemen and Precinct Committeewomen. Each county (at least in Florida) also selects one State Committeemen and one State Committeewomen to represent the county at the State Party level. The State Party selects one National Committeeman and one National Committeewoman to represent the State's interests at the Republican National Committee. The RNC then gets to change rules at their convention at will to suit the whims of whomever is currently on the committee, hence Ron Paul delegates being disenfranchised and not getting a nomination speech.