Theoretical Information

An interval is the difference between two pitches. A semitone is the closest distance between sounds, that is, they are adjacent keys. One tone is equal to 2 semitones.  

Intervals can be called Major (M), minor (m), Perfect (P), Augmented (A) or diminished (d)

An interval may be described as horizontal, linear, or melodic if it refers to successively sounding tones, such as two adjacent pitches in a melody,
and vertical or harmonic if it pertains to simultaneously sounding tones, such as in a chord.
Perfect unison  — sounds are located on one  scale tone and  sound the same (P1).*
Second (2nd)- an interval, the distance between the sounds of which is 2 degrees —  halftone (minor second) (m2)  
 or tone (major  second) (M2)
Third   (3rd)- an  interval consists of three  degrees  — one and a half tones (minor) 
or two tones (major)
Fourth   (4th)  — an interval of two and a half tones (4 scale tones) — a  perfect fourth.
Fifth (5th)- an interval of three and a half tones (5  scale tones), called the  perfect fifth.
Sixth   (6th) — an interval including 6  degrees: a minor sixth consists of four tones, 
major — 4.5 tones.
Seventh (7th) — an interval of seven  degrees  — five tones (minor Seventh)
or five and a half tones (major  seventh ).
Octave — an interval of six tones (8  degrees ).

* Examples are given in a harmonic presentation
More info Wikipedia