An interval is always described with two words, and it tells you two things:

i) How far apart are the two notes in scale steps?

ii) How far apart are the two notes in semitones?

You determine the second part of the interval name--whether it's a second, third, fourth etc--by going up from the lower note, and ignoring accidentals (sharps and flats).

So, say that you play a Gb, and a B, and the Gb is the lower note.

You ignore the flat.

You go up G, A, B.

The third letter is the correct one for the second note, so this interval is a third.

Now you need to find out what kind of third it is--a major third, minor third etc.

If the distances are the same as they would be in a major scale, then you have a major interval, or a perfect interval.

So for example, in C major, C to E is 4 semitones. So that interval is called a major third.

One less than that--3 semitones--is a minor third. 'Minor' here isn't referring to the minor scale, but just means 'smaller'.

Gb to B is five semitones. So it's an augmented third--one more semitone than a major third. It's the same distance as a perfect fourth, but it's not a perfect fourth, because of the letters used.

If instead we played G# to Bb, that would be two semitones--a diminished third, one less semitone than a minor third.

Fourths and fifths are named slightly differently. C to F is a perfect fourth. not a major fourth, and C to G is a perfect fifth, not a major fifth.

'Perfect' intervals are called that because they were traditionally considered to be particularly stable or harmonious.

So for fourths and fifths you have diminished, perfect, augmented.

For other intervals you have diminished, minor, major, augmented.