The Western Desert occupies a huge area of Egypt, on the side of the Nile that symbolized death. The western lands were where the sun god Ra went to die each day, and where the souls of the pharoahs went following their emtombment. It's harsh and deserted... but contains a handful of oases.
roughly from north to south:
Egyptian is spoken in all of the Western Desert. In Siwa, a Berber language is spoken, but nearly all Berbers in Siwa will also understand Egyptian Arabic. The Arabic in the Western Desert is slightly different from that near Cairo (e.g. ج, or "j", is pronounced as "zh" as in most other Arabic dialects, rather than as a soft "guh" as in Cairo and the delta) but it will not present any particular difficulties for someone who speaks Egyptian Arabic.
The central oases are largely a farming community, and thus English is not widely spoken except by the few people directly involved in tourism.
The oasis towns can be reached by public buses, but once on the spot, a 4x4 is necessary to visit any of the major sites. Experience driving in deep sand is necessary, or, more likely, you will need to hire a guide. Expect rates to be around US$100-$150/car/day which will also include all meals and supplies. Note that all tour companies for the Western Desert can also arrange private car transfer to/from Cairo to Bahariya, which will run you around US$40 each way.
Note: due to instability in Libya, anything west of the main desert highway running roughly north–south from Dakhla Oasis to Bahariya Oasis is closed by the military until further notice (2018). This also includes the road directly from Bahariya to Siwa, which is a closed military zone. While Siwa is open for foreign tourists, it can only be reached by driving a long way around around through the coastal road, and thus is probably a separate trip for most people than a visit to the central Egyptian oases.
By far the most famous natural site in the Western Desert is the White Desert, an ancient seabed that has long since turned into desert, and has left behind fantastic chalk monuments. The White Desert is roughly 40 km north of Farafra and can be reached by 4x4. Self-drive is allowed and going with a single vehicle is safe but you must know how to drive in deep sand and have sufficient supplies in case of emergency. Despite its remoteness, cell phone reception is generally available in the area and you are likely to get 2G internet as well. However, it is generally a better idea to hire a guide and driver. Take note that no facilities of any kind are available in the White Desert. There is no entrance fee or road, but it is only a few kilometers east of the main desert highway. Take note that as of 2018, the part of the White Desert that is west of the highway is closed by the military; it would be extremely dangerous and ill-advised to disregard this warning.
The Black Desert is an area of volcanic cinder cones roughly halfway between Farafra and Bahariya and is directly on the main highway. Climbing the cinder cones is a popular activity and gives a good view of the blasted landscape of the Sahara.
Wadi al-Hitan, which can also be visited from Fayium, is roughly the starting point where the Fayium oasis area turns into the Western Desert. This is a UNESCO world heritage site and is another part of the ancient seabed that is now the exposed Sahara, and which is famous for its abundance of large whale fossils. A new visitor center and museum has been built there (2015) which is up to modern standards. An entrance fee of 40 EGP (±USD $2.50) is required for Wadi al-Hitan and basic facilities are available (e.g. clean bathroom, small cafe).
There a handful of small springs of no particular note in the area which can be visited, most of which support farms. One previously notable spring was the "Magic Spring" (Ain el-Serw) which unfortunately went dry in the summer of 2018 after being an active natural spring since at least the Greco-Roman period of Egyptian history.
There are a number of other interesting but smaller areas, such as the Agabat Valley, which is a few dozen kilometers north of the White Desert and also has notable and far larger (but far fewer) chalk monuments, including one with a large "window" in it. Near the Black Desert, the "Crystal Mountain" can be found, which looks like any other hill made of ancient magma until you get closer and find that the rocks shimmer in the sun, and are in fact all crystals. Note that some people are disappointed when their imagination of "Crystal Mountain" does not meet the reality even though it is an apt description ("Crystal Hills" would be perhaps more apt).
There are also many historical sites in the central oases district, particularly the Valley of the Golden Mummies near Bahariya and the ancient town of Al-Qasr at Dahkla oasis. While the sites themselves will not impress anyone who has been to Luxor and Aswan, the area is nearly devoid of tourists, and consequently it may feel like a more personal experience rather than part of a tourist conveyer belt.
The Great Sand Sea and Gilf el-Kebir (where the "Cave of Swimmers" is found) are the two main sites far west of the desert highway, but both are no longer accessible since the war in Libya and remain closed as of 2018. The Great Sand Sea is a major smuggling route between Libya and Egypt and vehicles in that region without special authorization will be bombed with little warning. That is, unless you have a military escort and are part of the BBC or something, you are unlikely to access either area.