A musical mode is a series of intervals making up a scale of notes. Western music is based on these different orderings of scale which were brought to us from the Greeks. Heavily influenced by Pythagoras and his scientific analysis of harmony and scale, the Greeks adopted 7 forms or modes with different intervals. The mode was originally a scale of 5 notes or sometimes 6.
Some time in the 5th century AD, 4 of these modes were adopted by the church as 'Authentic' modes. At the time of Pope Gregory, 4 more were added ('Plagal' modes) and later still 4 more. The modes were used in plainsong where different notes had a particular significance. The first note (tonic) was a finishing note. The fifth of the scale (the Dominant) was the reciting note. It was in the 16th century that Henricus Glareanus produced a book adding the final 4 modes and giving them what he thought were their correct Greek names. Although these are acknowledged as incorrect, they have become accepted.
The remaining 4 modes added:
In modern times, the remaining modes are handily summarised by the following tables which shows the name of the mode, the original starting note of the mode to get the intervals and the pattern of intervals. In all cases, they are the scale formed by playing only the white notes of the Piano or Keyboard if you start on the given (tonic) note. Of course the intervals can be reproduced for any starting note but that would involve sharps and flats (black notes).