All scales in Western music begin on a note and end on its octave (i.e. from C4 to C5). The vast majority of scales follow the same pattern when they ascend as they do when they descend. When reading music, the musician is aware of what scale the song is based on by examining the key signature and the first few notes. Some songs use multiple scales. The technique of changing key signatures (and ultimately scales) is called modulation.
Chromatic scales are played by hitting all 12 half steps between a note and its octave. While the chromatic scale is the basis for all other Western scales (it uses all 12 tones present in Western music), it is rare for this scale to be used in the creation of popular music. Sometimes chromatic passing tones are used in modern songs, but because of the jarring nature of this scale, it is not particularly useful in the writing of music.
Major scales are made up of seven different notes. The major scale is created by using five whole steps (W) and two half steps (H). The pattern for the major scale is W-W-H-W-W-W-H. This pattern will create a major scale no matter what key you are playing in. Major scales are the most used scales in Western music. In the key of C major, the scale reads C-D-E-F-G-A-B. People tend to associate major scales with happy music.
Minor scales are seven note scales derived from major scales. A minor scale starts on the sixth note of a major scale, and then runs through the notes of that major scale until the sixth note (the root in this case) is hit again. For example, in the key of C, the sixth note is A, therefore, A minor reads A-B-C-D-E-F-G. The minor scale is usually considered sad sounding.
Harmonic minor scales are minor scales in which the seventh note of the scale is raised a half step. In the key of A minor, this scale would read A-B-C-D-E-F-G sharp. This scale is used to facilitate chord changes in minor keys. The raised seventh gives this scale an exotic feel.
The melodic minor scale is the most prominent scale in which the ascending pattern is different than the descending pattern. Ascending, the melodic minor scale follows the natural minor scale until the sixth and seventh notes, which are both raised a half step. Descending, it is the same as a natural minor scale. In A minor, it is spelled A-B-C-D-E-F sharp-G sharp-A (octave; beginning of descent)-G-F-E-D-C-B-A. This scale is used mostly in Jazz.
Major and minor scales (including harmonic and melodic minor) are the dominant scales in Western music, but many others exist. Pentatonic scales are five-note scales often used in the blues and rock and roll. The most common pentatonic scales are major and minor scales in which the half steps have been removed. Modes, other commonly used scales, start on different notes of the major scale, but continue to follow the major scale's pattern until the root note's octave is hit. The minor scale is an example of a mode. These are just a small sampling of the scales available to modern musicians.