There are THREE issues in guitar tuning. The first is the issue of equal temperament, that you should read first.

The need to temper the intervals is usually cited as the problem in guitar tuning. In fact, it is only one of the THREE problems. The other two are:

String inharmonicity

String falseness

We will deal with these in due course. Firstly, it is important to understand the principles of temperament in open string tuning. 

To overcome the existence of the Pythagorean comma the intervals on conventional musical instruments in modern Western music, are not tuned in accordance with harmonic ratios, except in the case of the octave.

Rather, the tuning system adopted is called equal temperament. The premiss of equal temperament is to divide to octave into 12 equally sized semitones. This can be achieved by narrowing all the perfect fifths by 1/12 of a Pythagorean comma, and widening all the fourths by 1/12 of a Pythagorean comma. This will give equal sized semitones.

Alternatively, we could look at this as dividing the octave into 12 equally sized semitones, which happens to produce tempered perfect fifths that are narrower than their harmonic size, by 1/12 of a comma, and tempered perfect fourths that are wider by 1/12 of a comma.

Either way, if all the semitones are to be an equal musical size, then all the fifths must be narrower than their harmonic ratio, by 1/12 of a comma, and all the fourths must be wider by 1/12 of a comma. On the guitar, this tempering of the fourths is especially important because there is a tempered perfect fourth between most of the adjacent strings.   

A consequence of the equal temperament strategy, is that all intervals of any one kind, are the same musical size.

Another important consequence, from the point of view of guitar tuning, is that all the major thirds will turn out to be wider than their harmonic ratio by about 2/3 of a comma. This becomes important in guitar tuning, because there is a major third between the G and B strings.

In the page on Pythagoras and the guitar, we did not mention the harmonic ratio for the major third. The harmonic ratio for a major third is 5/4, which means that if a string is stopped at 4/5ths of its speaking length, the new note produced will be a harmonic major third higher than the open string.

The 4th fret on the guitar does not stop the string at 4/5ths of its length, because the guitar is equally tempered, so its major thirds are all wider than the harmonic major third. The 4th fret is therefore a little closer to the bridge than 4/5ths of the length up the string towards the bridge.

Now let's look at Tuning open strings on the guitar