Your goal is to create a Desert surrounding your regular tiles, placing Desert Attractions of the different types (Oasis, Pyramid, Sphynx, and Obelisk), connecting them to your regular routes through Desert Stations, and placing Camels to bring people seeing the attractions and gain a few extra points as well.
There's a special rule to follow when placing tiles in this expansion: When you place the first Desert tile of the game, it can be placed freely. All following tiles (either regular or Desert) must be placed so that at least 1 side is connected to a tile of the same kind.
Desert tiles feature regular Car and Traveler pin points, but they lack Train pin points. In their place, they feature Camel pin points. You can use a Train Placement token to place a Camel on such a pin point, instead of placing a Train pawn. Desert tiles also feature Desert Trails: these are special routes that can be connected to both Highways and Railways: when you do, you place a Desert Station on the joining point token to mark the connection. Desert Stations are important for the scoring of both Desert Attractions and Camels.
When you place a Camel, you score 1 point for each Desert Station it connects to. Additionally, at the end of the game you gain points for your sets of different Desert Attractions that are connected to at least 1 Desert Station: 1 point for a singl Attraction, 3 points for 2 different Attractions, 6 points for 3, and 10 points for full sets of 4 different Attractions (the same Attraction can't be part of different sets, of course, but the Attractions of a set don't need to be connected to the same Desert Station nor to each other).
Unsurprisingly, the Desert Expansion also comes with a few Objectives: for example, the Desert Market must be part of a route with at least 5 connected Desert Trail segments, while the Great Oasis must be surrounded by at least 4 Desert tiles... but it also counts as an Oasis Attraction!