The Minor Scale is one of the most popular scales in Western Music. A minor scale is often used to compose a melody over a minor chord or to solo in a minor key. A minor key typically sounds much sadder or gloomy than the much more happy or energetic major scale. To give you some quick references "You Are My Sunshine" is in a major scale while "We Three Kings" is in a minor scale.

In the key of a minor the notes are A-B-C-D-E-F-G-A. If you noticed these contain the same notes as the key of C Major C-D-E-F-G-A-B-C but because they are in a different order they produce to different sounds and chords to be played over those songs. This can be a difficult concept to understand if you are just starting off learning music theory. I wouldn't not put to much thought into and come back to it after you understand many different scales and have a better grip of music theory.