Baliage is a technique for coloring or highlighting the hair in which the color is painted on in such a way as to create a graduated, natural-looking effect.
This technique gives the colorist the freedom to place color where she/he chooses, unlike foils which are more precise.
Baliage is great for coloring grays because the stylist can cover the few gray strands rather than having to color the entire head thereby reducing the need to come back for retouches as often!