A chord is a group of three or more notes sounding simultaneously, though they can be also found in 'arpeggio' or 'broken' form, where the notes are played separately, yet still considered as part of a chord. Chords are present in many musical traditions of the world, and constitute an important branch of study in western music. They are the most common means of accompaniment for the guitar.
On the How to play chords section you'll find all the information you need to be able to follow any song, from the very basic open chords to the most difficult barred ones: you'll learn everything about dominant seventh chords, suspended chords, power chords, and how and when to use those. On the Chord Theory section you'll find what lies behind those names, and how to better understand what you are playing.