An interval is a measure of the distance between two musical notes. These distances are usually measured in semitones (half steps) and tones (whole steps). Intervals may be melodic (two notes played consecutively) or harmonic (two notes played at the same time).
The easiest way to understand interval distances is to measure them against the chromatic scale. Here once again is the C chromatic scale. As before, scale degrees relate to the natural notes and the flat notes.