A plug-in hybrid vehicle will have a combustion engine powered by petrol or diesel, and an electric motor powered by a battery. Most hybrids or 'PHEVS' (plug-in hybrid electric vehicles) require their batteries to be charged by plugging-in to a power source. In some models the combustion engine will charge the battery, which it then uses for short journeys in full electric mode.
Some hybrids work the other way around: the battery and electric motor will support the engine, adding power when required. This is where the wheels are connected to both the combustion engine and the electric motor.
Plug-in hybrid vehicles are a compromise because they carry both technologies and the weight of fuel and a battery. For this reason they are also more complex. You will not get the efficiency savings of a pure battery electric vehicle and a hybrid also costs more to maintain than a fully electric vehicle. Plug-in hybrids can, however, reduce tailpipe pollution when used for short 'battery only' journeys.