all of music theory is based off the major scale. The major scale contains 7 notes ( Cmaj.= C D E F G A B C )
A mode is the major scale starting and stopping on a note other then the root. (D Dorian/ the 2nd mode of C major= D E F G A B C D)
Look under the thread entitled Music Theory Links, I believe Christoph posted a link there you may want to check out.