The term "chord" usually applies to three or more notes played together. However, in Rock and several other styles, two notes played together are also often called chords. This especially applies to the following formations which are called power chords, or 5th chords. The symbol for these chords consists of a letter name and the number 5 (E5, A5, D5, etc). The letter name (which is also the root note) refers to the first note of the chord. For example, 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.
The following example uses the power chord shapes you have just learned. Notice that the root notes of these chords are simply the open A, D, and low E strings. To change between these chords, all you have to do is move your first finger between the 5th, 3rd and 4th strings. Use all downstrokes with the pick as you play this example.