The most basic type of chord is a triad, a chord made of three notes built from stacked thirds. Each triad contains a chord root, and notes a third and a fifth above the root.
Basic chords:

Seventh Chords:
If we stack another third onto any of the basic chords, adding an interval of a seventh above the root, we now have Seventh Chords.
These chords names are commonly abbreviated, such as M7 for Major-Major Seventh and 7 for Major-minor seventh.
Other Seventh Chords:

Learn about Extended Chords
Learn about Chord Symbols