Balayage is a common hair-coloring technique in which a hair lightener is 'swept' onto the hair to create highlights. Balayage is actually the French noun meaning ‘a sweep’, and is pronounced nearly the same in English as in French, beh-lee-'auzh (IPA: /bɑ leɪˈɑʒ /).
In balayage the highlighting dye is painted on in a less patterned, more graduated, and often more integrated layer of highlights, most often blonde over brown. It is often said that it creates a more natural-looking effect, but a quick image search shows that many people are not, in fact, aiming for natural with their balayage. However, it is nearly always focused on natural-looking colors that layer in a not-too-obvious fashion. The goals was presumably, originally, to give a sun-bleached look to the hair, and that is still the most common effect.
While there are a wide range of variations, the more natural-looking balayage hair styles would probably fall under the description dirty blonde, and, indeed, they are often advertised as such. Balayage is strongly associated with lightly waved hair, longer hair, and Caucasian hair, although these are not defining features.