What is a Plug in PHEV Prius??

All current (stock) Prius do not plug in, so they are just HEV. (Hybrid Electric Vehicles)

There are aftermarket kits to add more batteries and a plug in charger control, to make current Prius able to plug in. They tend to change handling and be non-seamless in operation.

Toyota is working on a factory Plug in Hybrid Electric Vehicle, due in 2012. Ideally the handling will be less effected and the control will be seamless.

 

2012 Toyota Prius Plug-in: A modest route to the electric future - Boston Overdrive - Boston.com

It's a follow-on for the G3 with a larger battery, and the ability to charge that battery externally. If you read the attached review (a little dated, but it hits the high points...) it will answer most of your questions.

 

In a regular Hybrid you use a combination of electrical power from a battery and chemical energy from the gas to propel the vehicle in the most efficient manner possible (electrical generally being more efficient at low rpm high torque applications and gas at low torque high rpm). However, the battery in a conventional hybrid is charged primarily by the gas engine (and somewhat from captured regeneration), so while the hybrid is very efficient, it is 100% dependent on gasoline as a power source.

In a plug in hybrid the battery is charged by the power grid when the car is literally "plugged in", so the source of the electrical power is determined by the available power source (solar, coal, nuclear, whatever), so it decreases the dependence on gasoline a little bit. Once the initial battery charge is depleted during what is referred to as the all electric range of the PHEV then the car functions more or less like a normal hybrid.

There are some differences of design between manufacturers, but that is the general idea.