Interval distances are measured in numbers. The common names used in describing intervals are: Unison, Second, Third, Fourth, Fifth, Sixth, Seventh and Octave. These names are determined by the number of note letter names between the two notes being measured, including the starting and finishing notes.
For example, when ascending, C to D is a 2nd, C to E is a 3rd, C to F is a 4th, C to G is a 5th, C to A is a 6th, C to B is a 7th and low C to high C is an octave. Because of the possibility of each note being sharp or flat, there are various types or categories of each interval.
An example of this would be C to Db (minor second), C to D, (major second) and C to D# (augmented second). Because each of these intervals contains the letters C and D, they are all described as 2nds. C to Eb would be described as a 3rd.