Scales are yet another tool for the guitarists. I will not develop, I suppose everyone has a clue about what a scale is: a set of different notes separated from one another by different intervals. Scales are not so much used in Manouche style. Usually, arpeggios will be preferred because they naturally sound better due to the fact that they target the underlying chords notes, which is not the case with scales that bring alterations without really targetting any note in particular. Scales therefore become very useful when playing out of the fairly strict arpeggio canvas and adding alterations.
You should find proper definitions for these scales on the Web. In this section, I'll focus on workouts with scales, and specific tricks specificly for guitarists.