Batteries are classified by voltage and amp hour. The voltage can be thought of as the size of an engine, this determines the power output. A 48V will be more powerful than a 36V.
The amp hour can be thought of as the size of a gas tank. The higher the number, the greater the range. A 17.5ah has a longer mile range than a 13ah battery.
*Safety note: Battery voltage should be compatible with the controller and charger you are using. For example, only use a 48V charger with a 48V battery. Using an incompatible charger or controller could damage your battery, bike, or even cause an electrical fire.