As you may know, the Toyota Prius is fitted with an eCVT (electronic continuously variable transmission) unit that is using a single planetary differential gear set to combine power from two electric motors and a gasoline engine.
This system is much more simpler and efficient than a multi-speed gear-shifting transmission as it uses about 20 moving parts instead of 100. Also, due to the smaller number of moving parts, these types of transmissions have less chances of breaking down. However, don’t confuse the eCVT with the normal CVT that uses a friction belt reuniting two conic pulleys to change the gear ratios.
For example, the second generation Prius uses an electric motor for the wheels and one for the engine (which serves also as a starter and alternator) that are connected to the standard planetary gear set in the middle.