A basic chord is made up of only three notes e.g. C major chord = C E G. The number after the chord name represents any notes you are playing in addition to the three original notes e.g. C major 7 = C E G and B. It's called major 7 because B is the 7th note of the C major scale. C maj 7 should not be confused with C7 which is C E G and Bb (note that this features a flat seven).
Although it may sound confusing, once you know the basic rules it makes sense.
Here is a lesson on chord construction which should help.