The distance between two notes is known as an interval. We measure this distance using a scale. For example, the interval between C and D is known as a second because D is the second note of the C major scale. All interval have two names -- one tells the quality and the other tells the scale step. If you are measuring an interval using a major scale then you end up with the intervals shown on the first staff below. These interval measurements can being on any note of the chromatic scale. Each interval has a different character. They serve as the foundation of all harmony.

A melody is a consecutive series of notes. The intervals between those notes tell us a great deal about the character of that melody. Chords are created when three or more notes are played simultaneously. The intervals between those notes tells us a great deal about the quality of what we hear, whether dissonant or consonant