I never realized none of you know much about theory at all .... every answer so far is wrong. Some are kind of close.
Harmony is any time you have more than one note. It can be any two notes.
A chord is any time you have 3 or more notes at the same time.
ABCDEFGA
CDEFGABC
This is the difference of A minor and a third up ..... it can be major or minor. You just move up 3 notes.
ABCDEFGA
DEFGABCD
This is the difference of A minor and a 4th up.
ABCDEFGA
FGABCDEF
This is the difference of A minor and a 6th up.
These are the most common used harmonies used on the guitar. I charge $30 a hour for lessons ..... but I well give a few free here if you guys need this explained more.
Feel free to ask what ever you want. I know theory pretty well.
If you do not understand what makes up scales I just as well may be speaking Japanese for many on the harmony subject..