A Tritone is equivalent to 6 semitones or half steps.
By themselves, Tritones have a rather harsh, dissonant sound.
Tritones go by many names:
The tritone is interesting because it exactly splits the octave into two parts.
An octave spans 12 semitones, therefore the tritone falls right in the middle because it spans 6 semitones.
In the Major Scale, tritones only occur from the 4th to 7th degrees, or from the 7th to 4th degrees.
That means in the key of C, they occur between B and F, or between F and B.
Any other occurance would mean sharps or flats would have to be added to the scale.