A mode is a group of tones derived from a PARENT KEY, or PARENT group of notes. One group of 7 notes has seven modes because each time you could start from a different note, thus viewing the tones in a different ORDER, which is really important. Order makes all the difference.
Imagine you were looking at seven toy blocks, left to right, labeled C - D - E - F - G - A - B. The first block is the home or rest (tonic) and is the most important - it names the mode and is the final resolution of ideas. If we moved the C block to the end (DEFGABC), D becomes the tonic and all ideas resolve to D. This is a simple idea of how modes are formed. Each time a block is moved to the end, a new note becomes home, yet uses the same notes around it.
What's more is that each note in the group has it's own identity, importance and individuality.
Let's take a look at this model to help define what we are talking about.
MODES OF THE MAJOR SCALE
Ionian Mode ------ DO RE MI FA SO LA TI DO
Dorian Mode ------ RE MI FA SO LA TI DO RE
Phrygian Mode --- MI FA SO LA TI DO RE MI
Lydian Mode ------ FA SO LA TI DO RE MI FA
Mixolydian Mode - SO LA TI DO RE MI FA SO
Aeolian Mode ----- LA TI DO RE MI FA SO LA
Locrian Mode ----- TI DO RE MI FA SO LA TI
DO, a deer: Each new mode starts on the next tone, i.e. DO to DO, RE to RE, MI to MI, FA - a long,long way to run. Next page shows it with letter names for the notes instead of the solfege notes shown here.