Here's a fairly well written description, from wikipedia:
"A crossover SUV (also called CUV for Crossover Utility Vehicle) or XUV is an automobile with a sport utility vehicle appearance but is built upon a more economical and fuel-efficient unibody construction.
The CUV nomenclature was created by automotive marketing departments to move away from the station wagon, which has declined in popularity, and the SUV, which has been stigmatized by some people in American culture as environmentally-unfriendly, over-sized, and wasteful with fuel. The word "CUV" or "Crossover" is not a ubiquitous term and is primarily used by people in the automotive industry.[citation needed]
The nomenclature's unpopularity may be due to the fact that some CUVs are compact- to mid-sized SUVs built with car drivetrains and suspensions (Lexus RX330, BMW X3), while most CUVs are actually station wagons or 5-door hatchbacks with truck-like characteristics such as elevated suspensions and upright seating (Volvo XC90, Ford Taurus X, Cadillac SRX).
In some cases, marketing departments may sometimes blur the line between vehicle body types."