Battery capacity is usually referred to as mAh or Ah. "mAh" (Milliamps per hour) refers to 1/1000th of an amp, e.g.: 1.0Ah = 1000mAh.
The available capacity of a battery depends upon the rate at which it is discharged. If a battery is discharged at a relatively high rate, the available capacity will be lower than expected. Therefore, a battery rated at 100 Ah will deliver 5 Amps over a 20 hour period, but if it is instead discharged at 50 Amps, it will run out of charge before the theoretically expected 2 hours. For this reason, a battery capacity rating is always related to an expected discharge duration, such as 15 minutes, 8 hours, 20 hours or others.
Generally speaking, the higher the capacity, the longer a battery will last per charge (for rechargeables) or per use (for non-rechargeables).