Electronic Acoustic Guitar Tuners
Most electronic guitar tuners have a needle, or an LCD representation of a needle to show you whether the pitch is sharp or flat. If it is pointing dead center that means the note is in tune. If it's to the left that means it's flat (low), if it is to the right that means it is sharp (high). They will usually have a little scale with the notes on it, these notes highlight as you play the string. Most of them will have a green light that flashes when your string is in tune, a red light will flash to the left it it is flat, and to the right if it is sharp.
You can't really determine the best guitar tuner, because it's different for everyone. If you've just started get an electronic tuner for acoustic guitar. The pitch pipe tuners aren't that accurate, and it's not easy if you haven't trained your ears on what to listen for. Because you'll be tuning an acoustic guitar you'll need one with a built in microphone, which all of them should have. I've never seen one without one. Probably the best one for beginners is the acoustic guitar auto tuner. Calibrated for the strings on a standard guitar. All you need to do is play it note, it recognizes it and tells you whether or not it is sharp or flat. You don't need to know about the musical scale it will only recognize the notes of the open guitar strings.
Clip on Guitar Tuner
Another type of tuner for acoustic guitar is the clip on tuner. This doesn't have a built in microphone, but it doesn't need one. It clips on the head of your guitar and recognizes what note you play by the vibrations that it picks up. It's very useful when trying to tune in a loud room. Many of the microphone tuners will be thrown off by other noises. The clip on is also nice because you can leave it clipped to your guitar in case you want to check it periodically. It also works for an electric guitar, which is an added bonus.
Learning How to Tune Your Guitar
Tuning your guitar is something you should learn how to do yourself and an acoustic guitar tuner can help you do that. If you'd like a more detailed explanation of acoustic guitar tuners, and see some of the top selling ones, you can find much of the information on sites that sell musical equipment. It will give you a good idea of what other people are using and how they like them. There are also a lot of websites that help you tune your guitar using your computer and computer microphone. The problem is you can't take those with you.