Most camera manufacturers offer rechargable lithum ion batteries with their cameras. These batteries offer long life and many useful recharges. With my Nikon D40, I can usually take well over 600 images without a recharge (which only takes 45 milutes).
If your camera takes AA batteries, then you should go to Wal-Mart and get rechargable Nickel Metal Hydride (NiMH) batteries. These are relatively cheap and last a long time. The best part is that you can buy multiple batteries for almost nothing, charge them, and shoot for literally weeks at a time with just a pocket full of batteries.
Nickel Cadmium batteries are also availible, but they are'nt nearly as god as NiMH batteries.
The battery's capacity is measured in MaH (milli-amp hours). That is, your battery will deliver that many milli-Amps for exactly one hour before it dies. More is better. So if you have the option between a 1500 MaH battery and a 2,000 MaH battery, buy the latter, unless there is a big price differential.
Battery technology is evolving all the time. I remember the days when an 800 MaH NiCad battery was a big deal. Things will only get better.
But remember, when your batteries finally die, you can't throw them in the trash. Not unless you want to eat Cadmium in the fish you buy at the grocery store. So dispose of them properly. Take them to Radio Shack, and they will dispose fo them for you.
