Most improvisation in mainstream jazz is based on chord progressions. The chord progression is the sequence of chords that harmonizes the melody. Usually each chord lasts a measure; sometimes two, sometimes only half. A fakebook will give the symbol representing a particular chord above the corresponding point in the melody.
Even more important than the actual chords, however, are the scales implied by those chords. An improviser, when playing over a D minor chord, whose symbol is Dm, will normally play lines built from notes in the D dorian scale. This section documents the various chords and associated scales used in jazz. Familiarity with note names and locations is assumed.
If your aim is to become a jazz performer, you should practice improvising lines based on all the scales presented here, and in all twelve keys. Otherwise, you may stick to just one key per scale, but you should still practice improvising over each chord/scale relationship in order to better recognize their sounds.