In modern music chord alterations are quite normal, but in the older days composers were very reluctant to change the quality of their chords. In the middle ages and Renaissance only the major and minor triad were in use, and also the dominant seventh chord which is a major triad extended with a minor seventh. Other alterations of the triad, like the diminished and the augmented triad, were forbidden.
Still, we want to show them to you, because together with the major and minor triad, and the dominant seventh chord, they form the 5 most simple chord qualities. In the next example they are built on root note C, and their chord symbols are shown underneath.
C C major triad C-E-G root+major 3rd+perfect 5th
Cm C minor triad C-Eb-G root+minor 3rd+perfect 5th
C7 C dominant 7th chord C-E-G-Bb root+major 3rd+perfect 5th+minor 7th
C- C diminished triad C-Eb-Gb root+minor 3rd+diminished 5th
C+ C augmented triad C-E-G# root+major 3rd+augmented 5th
If you want to know more about note names got to Music notes.
If you need more information about how to play these chords on the piano, go to Piano Chords.