Skis will either come as "system" skis that include binding or "flat" skis that do not include bindings.
Skis with Bindings offer convenience, matching the ideal binding to the ski. Generally beginner to advanced intermediate skis come with a binding as well as some advanced and expert all mountain and frontside skis. These skis do not come pre-mounted. See Ski Binding Mounting Service.
Skis without Bindings allow you the choice of a binding. These are usually powder, freestyle and all mountain wide skis where being low to the ski is important.