Interval
The distance between two pitches.
This concept is one of the most important concepts in music. An interval (as well as chords formed from them) can either be major, minor, augmented or diminished.
As discussed in lesson 1, triads are made up of major and minor 3rd intervals. A study of music theory would not be complete without an explanation of used intervals -- as they are the basis of all scales and chords. Even though musicians may not talk about them, they are still quite important -- even more so if one learns to read/write music.
Intervals beyond an octave are called compound octaves. Chords that call under this category are 9ths, 11ths and 13ths.
Pitch
The specific quality of a sound that makes it a recognizable tone. Pitch defines the location of a tone in relation to others, thus giving it a sense of being high or low.