In minor scales, the sixth and seventh degrees are often raised. Although it's only one scale with two movable degrees, traditionally, we speak of three types of minor scales.

Natural Minor

All notes in the natural minor scale appear with the same accidentals as in its relative major scale. This is why it is called the natural minor scale:

Harmonic Minor

The seventh degree of a minor scale is often raised. This creates a harmonic minor scale because raising the seventh degree is harmonically motivated. Raising that note creates a dominant or dominant seventh chord on the fifth degree of the scale:

Melodic Minor

Besides raising the seventh degree, the sixth degree may also be altered. The resulting scale is called the melodic minor scale. The primary 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:

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 raised degrees that ascend and descend.