A quick review: the distance from one fret to the next fret is a half-step. The distance between two frets is a whole-step.

An octave is a distance of 12 half steps. When we divide the octave into 12 equal musical distances, they are labelled as such:

1 half-step (or 1 fret) is a minor second or flat second.
2 half-steps (or 2 frets) is a second (sometimes called a major second).
3 half-steps (or 3 frets) is a minor third or flat third.
4 half-steps (or 4 frets) is a major third.
5 half-steps (or 5 frets) is a fourth.
6 half-steps (or 6 frets) is a sharp fourth or flat fifth.
7 half-steps (or 7 frets) is a fifth.
8 half-steps (or 8 frets) is a augmented fifth or minor sixth or flat sixth.
9 half-steps (or 9 frets) is a major sixth.
10 half-steps (or 10 frets) is a minor seventh or flat seventh.
11 half-steps (or 11 frets) is a major seventh.
12 half-steps (or 12 frets) is an octave.

The trickiest part of learning to apply this idea to the guitar is that frequently the two notes you are trying to "see" the interval between are on two different strings. So, get used to learning intervals on one string, then start to apply it to multiple strings. For example, if you understand the list of intervals I wrote above, you can see that the distance between the notes A and E on the bottom E string is 7 frets, and therefore an interval of a fifth (see image below).

It can more difficult to "see" the interval of a fifth if the A and the E are on a different string, though (see image below). This can get confusing because there is frequently more than one place to play a note on the guitar.

Learning to identify by sight and hearing all 12 intervals of the octave is an invaluable skill for any musician. It is called Relative Pitch Idenification. If you take nothing else away from these lessons in intervals, remember this: A half step is a distance of one fret and a whole step is a distance of two frets.