An interval is the difference in sound between 2 notes.A diad is a 2 note chord and a triad is a 3 note note chord.Triads are the basic building blocks of western harmony if you have an E note as the lowest sounding note with a B note as the next highest sounding note followed by another E note an octave higher you have an E5 chord also known in rock as the power chord much used by Pete Townshend,Van Halen etc it can also be called E5 no 3rd.

If you play the notes E and B together with E being the lower note they will sound an interval of a 5th.Two notes of the same pitch played together are called a unison if you raise 1 note by a semitone and played them together you have an interval of a minor 2nd.

If you keep raising one note by a semitone you will get consecutively a major 2nd,a minor 3rd,a major third,a perfect 4th,an augmented 4th/diminished 5th,a perfect 5th,minor 6th,major 6th,minor 7th,major 7th and an octave.When you tune your guitar by the 5th fret method you are using intervals of a unison(2 of the same note sounding simultaneously)to tune it.