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. An octave is a very important interval because it marks the amount of distance (literally the interval) at which the musical alphabet starts to repeat. So, 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.