A selection of notes arranged in ascending or descending order is known as a scale. One of the most common scales is the C Major scale...

e|---------------------------------7-8--|
B|----------------------------8-10------|
G|---------------------7-9-10-----------|
D|--------------7-9-10------------------|
A|-------7-8-10-------------------------|
E|--8-10--------------------------------|

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The major scale consists of a mixture of tones (2 Frets) and semitones (1 Fret) in a particular order. The order tones and semitones are arranged in a major scale is tone, tone, semitone, tone, tone, tone, semitone (or T,T,S,T,T,T,S). The note the scale starts on determines the name of the scale. For example, if you start on the note of C and ascend in the order of tone, tone, semitone, tone, tone, tone, semitone, you have the C Major scale...

_|___T__T_S_T__T_T_S__T_T_S__T_T__T_S___|
e|---------------------------------7-8--|
B|----------------------------8-10------|
G|---------------------7-9-10-----------|
D|--------------7-9-10------------------|
A|-------7-8-10-------------------------|
E|--8-10--------------------------------|
_|___T__T_S_T__T_T_S__T_T_S__T_T__T_S___|

This scale pattern can now be moved up or down the fret board so that you start the scale on different notes. For example, if you start the pattern on the note of A, you have the A Major scale...

_|___T_T_S_T_T_T_S_T_T_S_T_T_T_S___|
e|----------------------------4-5--|
B|------------------------5-7------|
G|------------------4-6-7----------|
D|------------4-6-7----------------|
A|------4-5-7----------------------|
E|--5-7----------------------------|
_|___T_T_S_T_T_T_S_T_T_S_T_T_T_S___|

A piece of music which uses the notes of a partiular scale is said to be in the key of that scale. For example, if a song uses just the notes in the E Major scale, the song is in the key of E Major, or in E Major. The note which the scale begins is the key note.