A triad is a series of three notes separated by intervals of a third.
This position, stacked in thirds, is called root position. In this position, the bottom note is called the root, the middle note is called the third (being a third above the root), and the top note is called the fifth (being a fifth above the root.)
A triad thus contains two intervals of a third: one between the root and the third, and another between the third and fifth. Since there are two types of third, this produces four types of triad.
If both the bottom third and the top third are minor thirds, you have a diminished triad.
If the bottom third is minor and the top third is major, you have a minor triad.
If the bottom third is major and the top third is minor, you have a major triad.
If both thirds are major, you have an augmented triad.
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