The distance in pitch between two notes, e.g. 'C' - 'F' is a 4th. The interval is counted from the lower pitch to the upper pitch inclusive, i.e. 'C' (1), 'D' (2), 'E' (3), 'F' (4). It is known as a 4th as it is the 4th note from 'C' (remember to count your starting note as 1).
A step by step guide to help you work out intervals.