Currently, there are four trains per hour between London Victoria and Brighton Mondays to Saturdays (off peak)
The two faster services (xx:00/30 from Victoria and xx:18/48 from Brighton) are the Gatwick Express extensions. These call only at Gatwick Airport and are formed of the Gatwick Express Class 387/2s. Prior to December 2015 (?), these services ran as Southern Services (departing Victoria at xx:06/xx:36, and Brighton at xx:49/xx:19), and called at East Croydon and Clapham Junction only. These services were formed of 8-12 carriage 377s, with 10 car 377/6s also being used, and, more interestingly, Gatwick Express Class 442s despite the service not calling at Gatwick.
The two slower services (xx:20/50 from Victoria and xx:28/58 from Brighton) are Southern Services. One train calls at Clapham Junction, East Croydon, Horley and Burgess Hill, whilst the other service calls at Clapham Junction, East Croydon, Gatwick Airport and Hassocks. Interestingly, all are fast through Haywards Heath. These services are the ones formed of Southern's 377 stock, which can throw up any combination of 4-12 car trains, with many 377/6 units being used, predominantly on Saturdays to form 10 carriage trains.
Personally I think some (not all) of these services should run to Eastbourne or Littlehampton to improve Coastway Capacity and reduce attaching and dividing at Haywards Heath but that's a whole other debate.