In minor scales, the VI and VII degrees are often raised. Although it's only one scale with two movable degrees, traditionally we speak of 3 types of minor scales.
In the natural minor scale, all notes appear with the same accidentals as in its relative major. Hence the name of natural minor:
The seventh degree of a minor scale is very often raised. The resulting scale is referred to as harmonic minor scale because the raising of seventh degree is often harmonically motivated. Raising that note forms the dominant chord or dominant seventh chord on the fifth degree of the scale:
Besides raising the seventh degree, the sixth degree may also be altered. The resulting scale is called the melodic minor scale. The main purpose of this accidental is to facilitate the melodic movement from degree sixth to the seventh degree, avoiding the augmented second that is formed in the harmonic minor scale. Therefore, it is referred to as the melodic minor scale:
In traditional music theory the melodic minor scale is presented with the raised VI and VII degrees when the scale is ascending while descending without the accidentals:
In real music we may find the raised degrees ascending and descending.