A minor key with the same notes as a particular major key. For example, C major contains the notes C D E F G A B. The minor key with this same key signature is A minor, so it's the relative minor of C major.
Basically, the minor scale is derived from the Aeolian mode of the major scale, and the aeolian mode contains the same notes as it's relevant major. To find the relevant minor of a major scale, u must take the 6th note of the scale (or 6th note in the 7 note intervals, or just 3 semi-tones back of the starting major position) and that becomes the root note of the relevant minor. Example: C major contains C D E F G A B, the relevant minor starts from the 6th note, which is A, so it's A minor, which becomes A B C D E F G. However, if u play an A minor or Aeolian C major over a Cmaj chord, it will sound exactly like the C major scale, the true effect is demonstrated when played over a backing track of Amin.

The interval for the major scale is: WWHWWWH (W: Whole step (2 frets)) (H: Half step (1 fret))
The interval for the minor scale is: WHWWHWW

Remember, the chord backing is more important to demonstrate a relevant minor of a major. If you want to improvise around a minor scale to make it sound minor, try the other minors, which are the harmonic minor and the melodic (minor).
Hope this helped...
relative minors are pretty simple i was in music theory and that was the first thing we learned... bascially in any MAJOR scale the 6th note of that scale is the relative minor..... for example...
C D E F G A B C Am
G A B C D E F# G Em
F G A Bb C D E F Dm
Bb C D Eb F G A Bb Gm