Lithium-ion Polymer batteries are rechargeable and can be made in a variety of shapes. They usually tend to come in a soft package or pouch, which makes them lighter but also lack rigidity.
Li-Poly batteries have about four times the energy of density of NiCd or NiMh batteries and are also lighter. Typically Li-Poly batteries are designed to be recharged in the device rather than in an external charger.
Some applications that use Li-Poly batteries are RC cars and planes, as well as PDAs, laptops, cell phones, MP3 players and GPS units.