Semitones and tones are distances from one key to another key on a keyboard. For example if we play middle C on the keyboard and then play the next white note to the right which is D then that distance is one tone. For those of you not familiar with a keyboard then get my Lesson 1 e-Book here, which will show a 61 note, keyboard with the correct notes names printed on each key.
The thing to remember when counting semitones and tones on a keyboard is to count the distances and not the notes. A semitone is the closest distance from one note to another note. So again, if we play middle C on a keyboard and want to move one semi-tone to the right the note you should be playing is C sharp that is the black note between C and D. When you first look, at the C and D keys they might appear to be closest to each other but the black notes are placed between the white notes, which make them closer. Now starting from middle C again if you move one semitone to the left, you will notice there is no black key between the white keys so the closest key is B.
Counting semitones on a keyboard is very useful especially for finding chords. For example if you wanted to find C chord which consist of a root, third and a fifth we use distances to find the notes. The distance from the root to the third is four semitones and the distance from the root to the fifth is seven semitones. Now since the root is always the name of the chord in this case C, count four semitones (distances) to the right from C to get E then count seven semitones to the right from C to get G and you have just found your C chord C, E and G and since all major chords (triads) use a root, third and fifth you can find all twelve major chord by counting semitones.
I personally count distances in semitones, that's just my preference. Instead of saying two semitones, you can of course say one tone. It's up to the individual. Remember to count distances and not notes.
About the Author
Michael David Shaw (Mike to his friends) runs websites http://www.mikesmusicroom.co.uk and http://www.keyboardsheetmusic.co.uk