Western music has divided the octave into twelve equidistant notes. The major scale uses seven of those notes. The minor scale uses a different seven (or nine, actually, but let’s not get into that). Such scales are known as heptatonic (seven tones).
Modern guitarists mostly get by with pentatonic scales, taking five notes from the minor scale to play blues, or five notes from the major scale to play country. Since rock borrows heavily from both blues and country, rock guitarists tend use either major or minor pentatonic depending on the song.
Jazz players? Well, they prefer strange seven and nine note scales with even stranger sounding Greek names. Good for them.
Skilled and inventive guitarists throw in lots of notes that don’t belong in the pentatonic scale - sometimes to harmonize, sometimes to give you a tasty blue note – but they rarely stray from those five basic notes. Most of the classic guitar solos follow this pattern.
Isn’t it amazing what you can do with a driving beat, a few simple chords, and five little notes?