Coop Rholang is the flavor of rholang promoted by the RChain Cooperative. It is currently the most popular flavor with the most stable interpreter as evidenced by its use in the Rholang Cheatsheet. The best source for info about Coop Rholang is http://developer.rchain.coop
Coop Rholang is notable for presenting the programmer with a distinction between names and processes ,and requiring explicit conversion between the two with the * and @ operators. It is also notable for including polyadic sends and receives.
An alternative but much newer and less mature flavor is Simplified Rholang