Just Intonation

In music, notes are defined by the their frequency, and their relationships to other frequencies. Since our ears are capable of recognizing frequencies in the 20 to 20000 Hz range with great accuracy, we rely on mathematical observations when devising a musical scale out of different frequencies. Two main types of assigning notes to frequencies are just intonation and equal temperament. In just intonation, the frequencies that define notes are related by ratios of small whole numbers. On the other hand, equal temperament defines all notes as multiples of the same basic interval, so that the distance between each frequency is the same for all.

Here is an example sourced from Wikipedia, it plays a pair of major chords with the first in each sequence tuned in equal temperament, with the second tuned in just intonation.

http://upload.wikimedia.org/wikipedia/en/9/99/Just_vs_equal.ogg