A self-charging hybrid uses the engine to charge its (much smaller) battery, as well as recuperating energy normally lost during braking. It can only travel for a very short distance (a few miles maximum) under electric power, but the hybrid system works with the engine to provide greater efficiency. A plug-in hybrid can be charged at home and will travel much further (20-30 miles generally) under electric power meaning it won't, in theory, use any petrol for short journeys. This is better if you cover lots of short journeys and can charge a car at home - although it's more hassle than a self-charging hybrid, and once the battery's flat it's lugging around a lot of weight in batteries which will impact fuel economy. This page might be useful: www.honestjohn.co.uk/should-i-buy-a-phev-or-hybrid