While most vehicles travel across the ground using wheels, tracks or legs, some advanced machines are fitted with anti-gravitic drives that enable them to swoop and hover a few metres above the battlefield. This is not true flight, but rather gravitic repulsion in a limited plane. The Space Marine Land Speeder is the only Imperial vehicle with this capability as the helio-prismic engines are very difficult and expensive to produce, limiting their issue to only the Emperor's elite troops. The Land Speeder is a highly manoeuvrable, light reconnaissance and fast strike craft. It is capable of high speeds and can execute very tight turns but is limited in its vertical movement by a climb/dive incidence of no more than 35° from the horizontal due to inverse flux anomalies which arise beyond this point.
The only race to have truly mastered skimmer technology is the ancient and highly advanced Eldar. They build nearly all of their vehicles around sophisticated tri-polar phased crystal engines. These are manufactured in all sizes from the small and super-light units in jetbikes to the huge engines that power super-heavy grav-tanks such as the Scorpion. The Falcon is perhaps the best example of Eldar skimmer technology. It is a large tank that is not only capable of low altitude flight but can also climb and dive almost vertically. Orks, on the other hand, are not sufficiently advanced to fully comprehend anti-grav technology.
These are vehicles that hover above ground level using anti-gravity drives, huge turbofans, supernatural energies, etc. Note that these are not full-blown flying machines. They generally hover at a few metres above ground level but are capable of making a powered boost to gain height and cross obstacles. Skimmers can ignore terrain as they move but if they begin or end their move in difficult or impassable terrain, they have to test to see if they are immobilised (as noted for ordinary vehicles above). Immobilised skimmers crash and are destroyed if they are moving more than 6" in their move. The player should specify their speed before testing for difficult terrain. An immobilised skimmer travelling slowly makes a controlled landing and may not move or turn again in the game.