One of the most basic questions asked by student guitar players is 'what is a chord'?

A chord is simply a collection of single notes that, when played together give a pleasing sound. Chords are made up of a minimum of three notes and are referred to by the 'root' note - that is, the dominant note that you hear.
Thus a basic 'C chord' will contain the notes C, E and G. When played on a guitar, there are six strings that can be played together so the notes can be repeated. A full C chord played on the guitar will therefore contain the notes G, C, E, G, C, E each from a different string (6th string to 1st string respectively). The notes that make up a chord are determined by the mathematical relationship between the notes in the scale.

By changing one or more notes within the chord you get different sounds and moods from the chord. The simplest variants are the seventh (e.g. C7), which gives a slightly harsh, accented sound and the minor (e.g. Cm), which gives a more subtle, melancholic sound often heard in ballads or love songs. There are many other variants that the student guitar player will eventually learn, including sixths (C6), suspended (Csus) and combinations of these (e.g. Cm7sus4).