Desert

Terminology
By definition, a desert planet is one that is mostly or completely desert - a world with a hot, arid climate and scarce precipitation. The main feature that determines if a planet can be considered a desert world is the lack of significant amounts of liquid water on the surface.

Water scarcity
It would be wrong to assume that water is completely absent from desert planets, however. It often subsists under the surface as ground water or inside permanent polar ice caps. On some desert planets tiny amounts of liquid water can also be found on the surface, which leads to the formation of swamp biomes. This is much more likely to occur on low elevations. The distinction between desert planet and terran planet is also not as absolute as one would think. Many dry worlds have had large oceans at some point in their geological history. Several factors can lead to desertification processes that will transform planets from acqueous into barren desertic worlds.
On some former terrae increasing amounts of water were subducted under the planetary crust, on others greenhouse effects initiated by luminosity changes of the parent stars have caused the evaporation of the oceans, which was followed by the photodissociation of water molecules contained in the atmosphere due to ultraviolet radiation. Another scenario that has been observed is the gradual loss of atmospheric density caused by charged particles that strip away atoms from the outer layers of atmosphere. This density loss leads to a drop in atmospheric pressure that makes the subsistence of liquid water impossible. The aforementioned scenario is especially likely to happen on worlds that have a weak magnetic field or no magnetosphere at all.

An example of the planet Mars' transformation from a terran world to......a desert planet.
Geomorphology
Depending on the intensity of weather phenomena reshaping the surface some desert planets still display some remnants of their "wet" phase, like riverbeds, dendritic valleys, outflow channels and gullies. Other desert planets, especially those that have lost their water reserves immediately after their formation, don't bear these traces. The morphology of their surface is mainly shaped by tectonic events, volcanism and aeolian erosion.

Habitability
Despite the harsh conditions that persist on the surface of dry worlds life is not unlikely to be found on them. In fact they often have a broader habitable zone than watery planets, as the occurence of greenhouse effects at high temperatures and the coating of the surface with water ice at low temperatures are much more unlikely to happen.

.View of Augen V, a very far away cool desert planet with colossal living beings.View of a desertic planet deemed the 'bowling ball planet'1. ↑ http://sciencing.com/desert-planets-23092.html
2. ↑ https://en.wikipedia.org/wiki/Martian\_polar\_ice\_caps
3. ↑ https://en.wikipedia.org/wiki/Future\_of\_Earth#Ocean-free\_era
4. ↑ https://en.wikipedia.org/wiki/Atmosphere\_of\_Mars
5. ↑ https://en.wikipedia.org/wiki/Water\_on\_Mars
6. ↑ https://en.wikipedia.org/wiki/Aeolian\_processes
7. ↑ http://www.astrobio.net/news-exclusive/alien-life-more-likely-on-dune-planets/