The term ‘chord’ is short for ‘conchord’ and means any two or more notes sounded simultaneously. During the middle ages composers began to experiment with different combinations of notes, initially doubling melodies at intervals a fourth or fifth, then eventually combining multiple melodies at once. It was out of these experiments that chords and the concept of harmony arose.
Over the centuries certain combinations of notes were found to be more pleasing than others, these came to be known as conchords. The less pleasing combinations are called dischords. What composers found is that notes an octave or a fifth apart were the most harmonious, while notes a third or a sixth apart were less harmonious but sweeter sounding. And so it is these intervals, the fifth, octave, third, and sixth that make up our standard chords today.
The theory of chords is somewhat more involved than described here and is very closely related to scales and intervals also but this short description should serve as a good introduction. To learn more about chords follow the links below.