Most DSLRs pack a battery that will last you all day, but compact point-and-shoot and MILC/CSC/EVIL don’t necessarily come with that luxury. When considering something of the more compact variety, you want to weigh both the longevity of the battery and the cost of a second one. Sometimes you can get a better camera with poor battery life, but the cost of an additional battery isn’t very expensive. If you don’t mind charging two batteries this can be a good option.
With DSLRs you’ll often get a good battery but sometimes that battery will perform better in certain circumstances. DSLRs do not require the use of the LCD screen and you’ll generally take pictures through the viewfinder. The battery will last much longer when the LCD screen is not powered, so companies will often provide two ratings for the battery life: one in the number of photos you can take and one in the number of hours the battery will last. The number of hours generally refers to the amount of time the camera can be actively functioning with the LCD screen turned on and the number of photos is simply how many pictures you can expect to take without the aid of the LCD screen. When judging battery life for a particular camera, be sure you know if you plan to use it more with the LCD screen on or off first.