Basically a mode is a type of scale, as in ‘doh re mi fa so la ti do.’ Alter just one of those notes and you can call the scale a mode. Each mode is believed to have begun on a different note of the scale, for example, C to C (Lydian mode) or E to E (Dorian mode) and so on. The seven main categories of mode have been part of musical notation since the middle ages. Ionian, Dorian, Phrygian, Lydian, Mixolydian, Aeolian and Locrian. Some of them are major modes, some are minor, and some are ambiguous. Some modes are sadder or holier than others.
The Ionian mode is a simple ‘doh re mi’ major key. It is the modern major scale. It is composed of natural notes beginning on C. A typical example of music in the Ionian mode would be Mozart's Flute and Harp Concerto in C major
The Dorian mode is very similar to the modern natural minor scale. The only difference is in the sixth note, which is a major sixth above the first note, rather than a minor sixth. Good examples of the use of Dorian mode in music are 'Billie Jean' by Michael Jackson, 'Smoke on the Water' by Deep Purple and the traditional Scarborough Fair.
The Phrygian is the third mode. It is also very similar to the modern natural minor scale. The only difference is in the second note, which is a minor second not a major. The Phrygian dominant is also known as the Spanish gypsy scale, because it resembles the scales found in flamenco music. Music that uses the Phyrigian mode include Liszt's Hungarian Rhapsody No.2,
The Lydian mode has just one note changed from the Ionian, a major scale, but with the fourth note from the bottom sharpened to give a slightly unsettling sound. Music that uses the Lydian mode includes Chopin's Mazurka No. 15, the theme tune to 'The Simpsons', and the third movement of Ludwig van Beethoven's String Quartet No. 15 in A minor.
The single tone that differentiates this scale from the major scale is its seventh note, which is a flattened seventh rather than a major seventh. Tunes that use the mixolydian mode include The Beatles' 'Norwegian Wood', the theme to the TV series of Star Trek and Debussy's 'The Sunken Cathedral'.
Aeolian & Locrian Mode
Aeolian is the natural minor scale, heard in such popular songs as Bob Dylan’s 'All along the watchtower' or REM's 'Losing my Religion'. The Locrian mode has five notes in its scale flattened a half-step. There are passages in the Locrian mode in works by Rachmaninov, for example the Prelude in B minor, op. 32.