Desert
Source GM Core pg. 93
Desert encompasses sandy and rocky deserts as well as badlands. Though tundra is technically a desert, it's classified as arctic, as the climate is the primary challenge in such areas. Sandy deserts often have quicksand and sandstorms.
Rubble
Source GM Core pg. 93
Rocky deserts are strewn with rubble, which is difficult terrain. Rubble dense enough to be walked over rather than navigated through is uneven ground.
Sand
Source GM Core pg. 93
Packed sand doesn't usually significantly impede a character's movement, but loose sand is either {rules 414 "difficult terrain"}} (if it's shallow) or uneven ground (if it's deep). The wind often shifts sand into dunes, hills of loose sand with uneven ground facing the wind and steeper inclines away from the wind.