For the beginner guitarist, harmonic intervals are the next step up from playing single notes. A harmonic interval (also called a double stop, dyad, or less commonly couplet) are two notes played at the same time. The distance between the two notes is called an interval.
A predominant type of harmonic interval know as the power chord consist of the root note of the chord and a fifth. An advantage to understanding power chords is their shape can be used to quickly determine the location of perfect intervals. This improves ones overall understanding of the location of notes on the fingerboard by relation to one another and builds the groundwork for understanding scales.
A chord is named after its root note, which is typically the lowest note. E.g. a C chord consists of the notes C E G, with C most likely to be the lowest note. Chords may be modified by “inverting” them, which means to reorder the pitch of the notes by raising or lowering them an octave, e.g. playing a C chord as E G C, which would be named CE. However, the general rule of thumb among guitarists is to refer to a chord by its lowest note.
Content Credit: http://en.wikibooks.org/wiki/Guitar/Intervals_and_Power_Chords