From Yahoo News: 

Prepare to Plug in for 100-mpg Hybrids

A new strain of gas/electric hybrid vehicles called plug-in hybrid electric vehicles, or PHEVs, represent the latest wave of fuel-efficiency-boosting technology being explored by carmakers and third-party firms. They promise to boost gas mileage to 100 mpg and beyond by running only on rechargeable electric batteries for limited distances. As the name suggests, they can be plugged in to recharge.

Unlike gas/electric hybrids on the market now, which can’t be plugged in, PHEVs require more batteries in order to run on electric power alone. So part of the process of converting an existing hybrid, such as the Toyota Prius, into a PHEV is to add more batteries, particularly newfangled ones like lithium-ion batteries, which are lighter and hold a charge longer than lead-acid or nickel metal hydride batteries.

(more here)