The term "chord" usually applies to three or more notes played together. However, in Rock and several other styles of music, two notes played together are often called chords as well.
This especially applies to the formations shown below which are called Power Chords, or fifth chords. The symbol for these chords consists of a letter name and the number 5 (E5, A5, D5, etc). The letter name (also known as the root note) refers to the first note of the chord. E.g. the root note of an A5 chord is A. The 5 refers to the other note which is a 5th interval above the root note. An interval is the distance between two notes. Intervals will be discussed in a later lesson. For now all you need to know is the root note of the chord and the fingering.
The example below uses the power chord shapes shown on the previous pages. Notice that the root notes of these chords are simply the open A, D, and low E strings. All you have to do to change between these chords is move your first finger between the 5th, 3rd and 4th strings. Use all downstrokes with the pick as you play this example.