When it comes to scales the main ones that are used major, minor, chromatic and pentatonic and they all have exact characteristics and an exact definition.
Major scales are probably the most commonly used in western music, whether it be classical, pop, country, gospel and even jazz. The major scale consists of eight notes, where the first and last note are the same note, just at a higher octave. The pattern of tones (whole step) and semitones (half step) is: tone, tone, semitone, tone, tone, tone, semitone.
Chromatic scales consist of 12 notes and each note is a semitone (half step) higher than the previous note.
Pentatonic scales consist of 5 notes that follow the pattern: tone and a half, tone , tone, tone and a half, tone.
When it comes to talking about a minor scale, there is actually 3 different types of minor scale. There is the harmonic minor, the melodic minor scale and the natural minor scale. These 3 different sorts of minors all have the same key signature, if started on the same note. For example D harmonic minor, D melodic minor and D natural minor all have the same key signature , which is one flat – B flat.
The difference between them can be described this way: the harmonic minor has the 7th note of the scale, raised a semitone (half step) as well as the key signature. The melodic minor scale has a rasied 6th and raised 7th note when it is ascending or going up and no raised 6th or 7th when the scale is descending or going down, as well as having the key signature. The natural minor has no raised or altered notes – it just uses the key signature.
Another way of explaining the difference is the pattern of semitones:
The natural minor is tone, semitone, tone, tone, semitone, tone, tone.
The harmonic minor is tone, semitone, tone, tone, semitone, tone and a half, semitone.
The melodic minor is tone, semitone, tone, tone, tone, tone, semitone when it is ascending and tone, tone, semitone, tone, tone, semitone, tone when it is descending.
I hope that this helps clarify the differences between the natural minor, the harmonic minor and the melodic minor scale/key.