A lithium polymer battery, or more correctly lithium-ion polymer battery (abbreviated as LiPo, LIP, Li-poly, lithium-poly and others), is a rechargeable battery of lithium-ion technology using a polymer electrolyte instead of a liquid electrolyte. High conductivity semisolid (gel) polymers form this electrolyte. These batteries provide higher specific energy than other lithium battery types and are used in applications where weight is a critical feature, such as mobile devices, radio-controlled aircraft and some electric vehicles.
Current lithium-polymer batteries are actually Lithium-Ion Polymer and perform similarly to Lithium-Ion batteries. The advantage however of Li-Polymer batteries is that they can be made much thinner - as thin as 1mm.
Generally, they don't suffer from the so-called "memory effect" common to NiCd and NiMH batteries.