Chords are groups of notes played together, that effect each other and produce harmonies, evoking feeling. Each chord has a root, or basic note, which the rest are placed on top of, and therefore give it it's name. A chord with a root of A is an A chord.
A guitar is designed so that multiple chords can be created easily, by having a wide variety of notes available wherever your hand is on the neck. 'Open chords' are made at the top of the neck, by holding down only three strings or less. E A D G & C are all 'open chords', and are the most common, and first chords you should learn. They are the building blocks upon which you can create any other chord!