Most "western" melodies are based on the Major Scale. The next most common scale which is used is the Minor Scale.

The Minor Scale is built from the 6th degree of the Major Scale.

Ie. If you write the notes from A to A (6th degreeof C Major) and use the Key Signature for C Major you will have an A Minor Scale

Since A Minor and CMajor share the same Key Signature they are "related."Related Minor and Major scales are separated by 3 semi-tones.

Ie. You can find the Relative Major of a Minor by going up 3 semi- tones from the Minor; and you can find the Relative Minor of a Major by going down 3 semi- tones from the Major.