This was one of the trending request for hairstylist last year. Ombré is a hair coloring technique that display a gradient effect from the roots to the ends. The style is generally darker at the roots through the mid-shaft and then gradually gets lighter from the mid-shaft to the ends. Some people get this look from growing out their dyed hair, others achieve this look from going to a salon getting balayage highlights.
Balayage is a free form highlighting technique. Powder lightener is generally used to paint individual strands of hair. This create a more natural look than normal highlighting. Many people go with this option to achieve the natural style that involves low-maintanence. If you're getting an ombré from the salon be sure to request balayage for a more softer highlight.
Interesting fact: The term "Ombré" is actually derived from a French word meaning "shade" or "shading."