Ombre hair refers to hair color fading from dark to light, or vice versa. Instead of the color starting from the roots, it gradually becomes lighter, or darker, towards the ends. The word "ombre" in French means shadow or shade. It can be thought of as having a gradation in the color of your hair. It can be done by bleaching the down side of your hair, or waiting until the dyed part of the hair moves down and new, darker hair grows.
Main celebrities with ombre hair are Sarah Jessica Parker in Sex and the City, Rachel Bilson, Whitney Port and Vanessa Hudgens.