In a triad, one note is called the Root, another is identified as the Third and the last note is called the Fifth. A major chord follows this formula without changing anything. A C Major chord would include a C as the root. An E would be the Third, since it is a third higher than the C, and G would be the Fifth, since it is five notes higher than the Root.
A minor chord is similar to a major chord discussed above, except the Third is taken down a half-step. So, in the key of C, you would have C, E flat and G.
In a diminished chord, the Third and the Fifth are both taken down a half step. In the key of C, that would be C, E flat and G flat.
With this chord, you start with your triad and you add the seventh note in the scale. In the key of C that would be a B. A Dominant Seventh chord would take that seventh note down a half step, so in the key of C, you add a B flat. According to Guitar Player World, these chords sound dissonant and often lead to a key change in a song.
People use different notations to identify chords. One is based on the regular alphabet, the other on Roman numerals. Here are a few examples. In the key of C, a C Major chord can be called a I (Roman Numeral One) or a "C" chord. A C minor chord would be identified as a small roman numeral "i" or small letter "c." In the key of C, if you were to make a major chord starting on a G it would be notated as a V (Roman Numeral Five) or a G. If you wanted a C Seven chord, that would be C7.