Different intervals have different qualities, as shown below:
Interval qualities can be best explained with the aid of a chromatic scale. If you look at the one below, it is easy to see that since intervals are measured in semitones, they may begin or end on a sharp or flat rather than a natural note.
Perfect intervals are 4ths, 5ths and octaves. If you widen a perfect interval by a semitone it becomes augmented (added to). E.g. if you add a semitone to the perfect 4th interval C to F, it becomes the augmented 4th interval C to F#. Notice that the letter name remains the same – it is not referred to as C to Gb.
If you narrow a perfect interval by a semitone it becomes diminished (lessened). E.g. if you lessen the perfect 5th interval D to A by a semitone, it becomes the diminished 5th interval D to Ab. Again, the letter name remains the same – it is not referred to as D to G#.
Major intervals (2nds, 3rds, 6ths and 7ths) become minor if narrowed by a semitone and minor intervals become major if widened by a semitone. A diminished interval can be created by narrowing a perfect or minor interval by a semitone. An augmented interval can be created by widening a perfect or major interval by a semitone.