Scales, keys, and the major / minor mood

A scale is a chosen subset of the 12 notes that 'belong together'. The most famous Western scale is the major scale - 7 notes that form the do-re-mi-fa-sol-la-ti pattern.

Pick any note as your starting point ('the tonic') and follow the major-scale interval pattern (W-W-H-W-W-W-H, where W = whole step, H = half step). That gives you a major key.

Major = bright. Minor = melancholy.

Change the third note of the scale to be one semitone lower, and the whole mood flips from sunny to introspective. That's the entire difference between a major and minor key.