Batteries are also a method of powering your robot, and is a common choice. These come in many kinds, and each has its benefits, and disadvantages. They have a special measurement known as the "C rating". This is the rate the battery can be discharged, as a multiple of the capacity. A 1A battery with a 20C rating can output 20 Amps, but not for long before it runs out.
NiMH: High capacity battery, limited power output, more stable than other kinds. Best for mild robots that don't move a lot of servos, and need to travel distances, or need reliability.
NiCd: Similar to NiMH, but different compound. Less stable, and can be damaged easily. Discharges 10% right after charge and sitting. Longer life cycle than NiMH.
Lead Acid: Heavy, high capacity, long charge time, Very reliable. Often used in large robots.
Lithium Ion- Polymer: High output amperage, upwards of 20x the capacity(C rating). Can explode if damaged or over charged. Best for robots with many servos, as high discharge can allow servo power surge without damage. Used in Ez-robots.
Those are the main kinds of batteries. Other kinds are available, but are less commonly used.