The set of musical tones that harmonize effectively in a given key. In the key of C major, these notes are:
C D E F G A B C
The note intervals (distance between notes, in whole tones) are as follows, with intervals noted between their corresponding notes:
Note C (1) D (1) E (1/2) F (1) G (1) A (1) B (1/2) C
So, the distance between D and E is one whole tone, but the distance between E and F is one half a whole tone, or a half tone.
A chord is made up of the first, third, and fifth notes of a scale, played simultaneously. A C chord is:
C E G
This chord is called the tonic, or root chord in C. There are 6 other diatonic chords (chords that use scale tones) in C. They are, with chord tones included:
Cmajor C E G
Dminor D F A
Eminor E G B
Fmajor F A C
Gmajor G B D
Aminor A C E
Bdiminished B D F
Get the idea? Every tone of the sequence of chords is a note of the C scale. Now, why are some major, some minor? The rule: if the interval (distance) between the first (D) and third (F, as in, the third note of a D scale) is four half tones (two whole tones), the chord is major. If the interval is three half tones (one and a half whole tones) the chord is minor. To restate: if you drop the third note of a scale by a half note, the resulting chord of that scale will be minor.
Now, look back up to the scale tone intervals. Again, we'll use D minor. What's the distance, in a C scale, between D and F? One and a half whole tones. So in the key of C, a diatonic D chord is minor.
chromaticblue has done a pretty good job of filling in the actual minor, major, and other funky scales on this very post. Check them out.