When purchasing batteries, its good to be aware there are two families of batteries: Regular/Normal and RC (radio control)
Regular/normal lithium ion batteries are what you find in your ipod, camcorder, phone etc. They are meant to last for over 500 charges, stay safe, and provide a C or two of current (For more details about **C** please read our basic battery tutorial)
RC lithium ion batteries are meant for radio controlled cars, planes, UAVs, quad copters, etc. They are meant to give a lot of power at once, up to 20C and are designed to never 'cut off' so that the battery will be damaged instead of having the plane fall out of the sky.
RC batteries are often raw cells, as seen here: There are two tabs coming out of the top and you see its labeled 20C for high current output capacity. They are often less expensive because there is no protection circuitry.