The Dorian Mode is the second Mode of the major scale. In the Dorian mode the third step (Eb, transposed to C) is a minor third away from the root note and therefore has a minor characteristic. The Dorian scale differentiates itself only by a half note (A — Ab) from the natural minor scale. Here are the scales for comparison.
The Dorian mode is used more often than the natural minor scale, since the half note higher sixth degree (A) sounds dissonant with a seventh chord (e.g. Cm7 = C Eb G Bb).
The three classical minor scales
The Natural or Pure Minor scale is the VI mode (Aeolian mode) of the major scale. Increasing the seventh degree by half a note will create the Harmonic Minor scale. Increasing the sixth and seventh degree each by half a note will create the Melodic Minor scale.