A Harmonic Minor scale is like the Minor scale, except the seventh note of the scale is raised a half step.
Where an A minor scale would be this; A B C D E F G, an A Harmonic Minor scale would go like this; A B C D E F G#.
Intervalically its spelled like this (T=tone, S=semitone)
T-S-T-T-S-T+S-S
That T+S is a tone plus a semi tone, or three semitones.