I recieved this tuner for free about 8 years ago when I got my first guitar and have been using it ever since.
It's a relatively low-end tuner, but it will work perfectly for a beginning or intermediate player. It's lot an LED screen with a needle that moves back and forth depending on how sharp or flat your note is. This tuner automatically knows which string you're tuning. When you've got the note right, a green LED lights up. There's a input cable for electric guitars for all you rockers out there. A nice bonus is the built in pitch pipe that will play E, A, D, or B so you have a starting point.
The only complaint I have about this tuner is that the sensitivity might be a little too high. You have to be pretty quiet when tuning, and even if you are, this tuner sometimes picks up vibrations from other strings or the guitar's body. It takes some getting used to, but it's not a serious problem.
It takes 2 AAA batteries, which will power it for a long, long time.