A guitar chord is made up of two or more notes, played on a guitar at one time. Chord voicings, that is the choice of individual notes in the chord, can be changed for many different purposes and playing styles.
The guitar is very capable and useful for playing chords; but it does have some differences with other instruments. Most guitars only have six strings. This means that for the very largest of chord-voicings it is often necessary to not use one or more tones from the chord. The layout of notes on the fretboard sometimes means that the notes in a chord are not played in tonal order, or make possible a chord that is composed of more than one note of the same pitch. Many chords can be played with the same notes in more than one place on the fretboard.
Guitars can vary both in the number of strings they have. They can also be tuned differently. Most guitars used in popular music have six-strings and are tuned: E-A-D-G-B-e. Usually the string with the highest pitch (the thinnest) is called the first string, and the string having the lowest pitch (the thickest) is called the sixth.