A scale is merely a series of tones that lie between octaves. That's not too much different than the definition of a chord. The main difference, however, is that a scale represents the foundation of a piece of music. A chord is essentially the harmonization of a scale.
The term exotic as it applies to musical scales is a very relative term. Usually, when people refer to a scale as being exotic they mean it is not commonly used. This may or may not be the case. It is difficult categorize scales because there are so many varieties according to number of tones, interval, and geographic origins.
While grouping scales according to their ethnic roots is useful to an anthropologist, it is not much help to a musician. A musician is interested in how a scale sounds and how it might be used in a piece of music.
Each scale has a unique sound but, like chords, they share certain family traits. The most useful way I have found to categorize scales is similar to how chords are categorize, that is, major, minor, dominant, etc. When grouped this way it is easier to translate their harmonic function.
The improvisor can use these scales to play over chords. For instance, if you saw a Cmaj7 chord on your lead sheet, you could play a major sounding exotic scale like the Hungarian Folk scale. This amounts to a key change without changing the tonal center and depending on the harmonic context can add unique tonal variety to your playing.
There are many different scale choices for each chord. The principle here is the same as with the Diatonic scale. Scales that have the same notes as the chord can be played over the given chord.
The scales listed here are grouped according to the chord function that they match. Each scale is matched with a chord as indicated to the right. The pairing of chord and scale is only one of several that can be made. Use this as a beginning point then explore pairings of your own.
These scales above have a major scale sound to them because they all have seven tones and a major third and a major seventh interval, same as the Diatonic scale.
These seven-tone dominant scales above are similar to the Mixolydian mode which has a major third and flatted seventh interval. The Mixolydian mode is the common scale to play over dominant chords. These modes all share a dissonances that allows them to be played with great effect over dominant 7th chords. These scales can also be played over many altered dominants.
These scales above have a minor scale sound to them because they have seven tones and a minor third interval along with a perfect fifth. All of these chords can be play over minor chords, however, some have a flatted seventh interval which makes them good to play over minor 7th chords. The other scales have a major seventh interval which means these scales could be played over minor(maj7) chords. Because the min(maj7) chord has a dissonant sound playing an exotic chord on top of them can be done very effectively.
Five-tone scales like the ones above have an open, airy sound to them. This is because of the large intervals between the notes. Often these scales will lack either a third or seventh interval or both. Tonally they are very flexible. In some cases you can play them over suspended chords. In other cases you can play them over major or minor chords. Many five-tone scales sound Japanese-like and are often called that indiscriminately. I've attemped to make a distinction here and apply the more specific name to them.
Six-tone scales can function like seven tone scales. Try the scales over the chords indicated here.
Eight-tone scales have a lot of notes in them. The bebop scales were developed by jazz musicians so the chord tones could be played on each beat of the measure while other non-chords tones, used as passing notes, could be played on the off-beat. With eight tones to the scale, and eight beats to the measure, each beat has a tone to go with it. Different bebop scales were developed to play over chords with different harmonic functions.
The Spanish Eight Tone scale was developed by Sephardic Jews in Spain in the Middle Ages. It's a great scale for Klezmer music.