Guitar scales are simply groups of notes that differ by various tonal relationships. In modern western music we have a total of twelve tones. Various permutations of these tones create different scale types. Each of the twelve tones are separated in pitch with equal intervals which we call half steps, half tones or semitones (one fret on the guitar). When all twelve tones are used together it is called the chromatic scale. If you play all twelve notes sequentially, the interval (pitch difference) between each note in the sequence will be the same. If for instance we start with C and end with C, we call it the C chromatic scale. If we start with G and end with G, it's called the G chromatic scale, etc.. No matter what note you start with, the intervals between each note remain the same, i.e., H H H H H H H H H H H H where each 'H' means half tone.
The distance between each fret on the guitar is a half tone. The distance between any two frets is called a whole tone. If we take the chromatic scale and skip every other note we create what is called a whole tone scale. This consists of six notes. For example:
C Chromatic Scale notes - C C# D D# E F F# G G# A A# B C
C Whole Tone Scale notes - C D E F# G# A# C (W W W W W W)
With the whole tone scale, like the chromatic scale, the intervallic distance between each note remains the same no matter what note you start with. In other words, the octave is divided equally. These kinds of scales are called symmetric scales. Other symmetric scales are the diminished scale with intervals of three half steps between notes and the augmented scale with equal note intervals of two whole steps. All other scale types are non symmetrical with the exception of the Tri-tone which splits the octave in two.
The two most common scale types are the diatonic scales which have seven notes and the pentatonic scales which consist of five notes. These get a little bit more complicated because being non symmetrical, their intervallic distances between each note will change whenever the scale starts on a different note. For example, the C major scale consists of the notes C D E F G A B. If however we start on the note D, the notes will remain the same throughout the scale but the intervals between notes will change.
The interval relationship of the notes in the major scale are ... WWHWWWH. Looking back at the notes in the C major scale we can see that if we start the scale from D then the intervals now become WHWWWHW. So the notes remain the same but the note relationship changes. Non symmetrical, diatonic scales therefore can be broken up into further scale types just by starting on a different note within the scale. These variations are what we call modes. For more on modes see Guitar scales and modes.
The major scale is used as a kind of 'base' scale for use in music theory. Chords, keys etc., are all explained by using the major scale as a reference. This in itself is another topic. To understand this, see the section here - learn guitar theory.