All digital SLR cameras come with a proprietary rechargeable battery that's capable of taking hundreds of shots on a single charge. For general all-around shooting close to home, that's probably fine. But if you plan on long days of shooting without time for a recharge or access to electricity, or you're photographing a once-in-a-lifetime event, you'll want at least one extra battery charged and ready to go.
Some SLRs also have compatible "battery grips." These attach to the base of your SLR, and give you a more comfortable handhold when taking vertical shots. As the name implies, they also provide additional power to your cam, either using your cam's proprietary battery or multiple "AA"-size batteries.