The term interval is what we use to describe the distance between two notes. While the theory of intervals do not play an important role in music per se, it is an important concept for describing and talking about music. Two notes in a melody or chord might be a certain distance from one another, and that would be true whether or not we had a way to define that distance, however, by using intervals we can communicate information about the relationship between those notes without needing to play them. If you were to mention the interval of an octave to a musician they would immediately know what you are talking about and even be able to hear it in their head.
Everything in music is intervals but the usefulness of understanding the theory behind intervals is in being able to communicate in words – and writing – information about music as well as being able to understand information about music that is being communicated to you.
Intervals have two main aspects; size and quality. The size of an interval describes how many letter names the two notes of the interval span. The interval C to G spans the letters C-D-E-F-G and is therefore a fifth. F to A is a third because it spans the letters F-G-A. (for more on letter names see the section on Pitch).
The introduction of accidentals (sharps & flats) in music means that you can have two different intervals that span the same number of letter names. D to B and D to B flat are both sixths because they both span six letter names, D-E-F-G-A-B. D to B flat however is a smaller interval because B flat is closer to D than B natural is. This is where we need to specify the quality of the interval. D to B is a Major 6th where as D to B flat is a Minor 6th.
Intervals are measured from the bottom up and can span anywhere from unisons (the same pitch) to fifteenths. After that, and in some cases before, we treat the intervals as though the two notes were in their closest position to one another (C to E is a third and so is C to the E three octaves and a third above the C). The different interval qualities are; Perfect, Major, Minor, Diminished, and Augmented. Doubly diminished and augmented intervals are possible also but are rare.
For more information about intervals follow the links below.