A kit is just a kit. You'll need to do all the assembly and then it turns into a roller.
A roller is an assembled kit - just add electronics (servo/esc/reciever) and a motor. Sometimes a set of tires and a body are included. Occasionally some spares or tuning parts are included.
RTR is a fully assembled kit with wired electronics, tires, body and transmitter. You provide the battery and off ya go. You usually won't see one of these for sale unless it was purchased originally as RTR.