Yes, I know I called the "tuning machine" a "peg", but I just don't know anyone who actually calls them "tuning machines". I think we call them "tuning pegs" from the history. That's what they are on the violin and cello and what they were on the lute and older guitars: just simple ebony pegs that held the tension by friction alone.
Back to those electronic tuners, here is what one looks like:
And you can pick one up here. Funny thing, though. I notice that every student I have had who uses one of these is tuned just a bit sharp. The only thing I can figure is that there is a little bit of lag in the response. By the time the indicator says you are at the pitch, you are actually just a bit above. Anyway, I still use one of the old-fashioned tuning forks:
No batteries! There are a few odd things about guitar tuning that plague beginners, which is the main reason for this post. Have you ever watched a guitarist spend minute after minute trying to get the guitar in tune? Going back again and again to the same strings, but never quite getting it right? This is very probably because he doesn't realize the difference between Equal Temperament and Pythagorean tuning. Here is a pretty good article that explains this in considerable detail. I think I can demonstrate it in fewer words. The problem is that equal temperament, which has been used in Western music since the 18th century, is different from Pythagorean tuning. Pythagorean tuning is based on the overtones and gives you nice pure intervals in some keys, but makes other keys hopelessly out of tune. Equal temperament solves this problem making all keys equally useful at the cost of making all intervals slightly out of tune.
Our hapless guitarist is caught in this contradiction because he is using harmonics (probably) to tune and then cannot reconcile the results with chords he plays to test. If he gets his E major chord perfectly in tune, the C major chord will be badly out of tune. I recommend something very close to the method used in the article on Equal Temperament tuning. That is, use your tuning fork to get one string exactly in tune. Paraglider suggests the fifth string, but I use the fourth string instead. Tune every string to the fourth (or fifth) string and you will not go far wrong. You may have to make some tiny adjustments, splitting the difference with a couple of strings. One big suggestion: never check your tuning with an E major chord! The third string is always going to sound a bit sharp. But if you fix it, then other chords will be out of tune. The reason is this: the sixth string has a very audible overtone that is the harmonic on the fourth fret. This is a Pythagorean G#, which is flat from an equal-tempered G#. If you tune the third string down to fix it, then it will be flat. The solution is to always check your tuning with an E minor chord!
I was going to be very clever and end this with a duet by Manuel Barrueco and David Russell introduced by saying "here are two guitarists who managed to get both guitars in tune!" But, alas, they did not quite manage it! So here are two other guitarists who are pretty much in tune. Julian Bream and John Williams with a transcription of "Clair de Lune" by Debussy: