Sossusvlei is a common tourist destination in the southern part of the Namib Desert, Namibia. The word vlei is an Afrikaans word that means "marsh" and Sossusvlei is in a small valley between the dunes which sometimes gets snow! The name of the 'town' (i.e. petrol station) is Sesriem, also the name of a nearby canyon.
Deserts, though they are very harsh, are a delicate ecosystem with a surprising amount of life living around and underneath the dunes. Keep this in mind when roaming around and driving in the area.
The Namib Desert is the oldest desert in the world and stretches over 1,500 km from the Orange River in the south into Angola in the north. There is a wide range of landscapes in the Namib, from gravel, to rocky mountains to huge dunes in varying colours of sand.
Sossusvlei is a salt and clay pan created by the Tsauchab river that flows through the Sesriem Canyon every 5 to 10 years. Even in very wet years it does not reach the Atlantic Ocean but drains away between the dunes of Sossusvlei.  Sossus means "place of no return" (note: there are other explanations, this is the one given by local guides).
The mud from the river stacks up at Sossusvlei and after some 1000 years the river searches its way through the next row of dunes. This is how the place called Dead Vlei was created, here the river used to drain away many years ago. Because of the lack of water all the trees in this valley have died, so the meaning of "Dead Vlei" becomes clear.
What makes the sight of the Dead Vlei so remarkable is that there is not even moisture enough for normal decomposition to occur. So all the trees here, though dead, have been nearly perfectly preserved for centuries.
Sesriem is approximately a 5 hour drive either from Windhoek or from Swakopmund, and Sossusvlei is roughly one hour from Sesriem, and inside the gated park area (open only from sunrise-sunset). It is not feasible to do a daytrip from Windhoek or Swakopmund; an overnight stay in the Sesriem area is necessary in order to visit and enjoy Sossusvlei. The roads to Sesriem are bumpy as soon as you leave the main highway (at Rehoboth if coming from Windhoek, and near Kuiseb Canyon if coming from Swakopmund). A high-clearance vehicle is better but not necessary, and the main roads from either direction can be traversed in a small sedan; you will just need to drive somewhat slower (e.g. 6-7 hr) and be on a heightened lookout while driving. If coming from the south (e.g. Aus) a higher clearance vehicle is even more recommended, though still not mandatory.
Sossusvlei is inside Namib-Naukluft National Park so a permit is required to enter. Permits can be purchased at the park office just inside the gate at Sesriem. The gate opens at sunrise and closes at sunset. The permit is purchased when leaving the park, not when entering, and as of April 2018 costs around N$80 for each foreign visitor plus N$10 per car.
The road from the accommodations in Sesriem to Sossusvlei itself is 65 km and tarred for all but the last 5 km. The speed limit is theoretically 60 kph, but everyone including the park guide drive at 100 kph. The last 5 km are through soft sand so you will need an all-wheel-drive to drive it. Alternatively, you can park your car at the end of the tarred road and either walk the last 5 km or take one of the shuttles that regularly run between the end of the tarred road and Sossusvlei (for a fee). You can also walk one way and take a shuttle the other.
Transport and guided tours to and from Sossusvlei are also provided by some of the lodges for their guests.
If you are self-driving, notice that there are two spots on the way where many guided vehicles stop: at the first dunes, and then a few km later at Dune 45. If you are there in the early morning when the gates open, you will want to bypass these stopped vehicles and just go directly to the end as Sossusvlei is best visited in the morning when the light and temperature are best.
The only place to get food, other than at fancy resort restaurants, is from the small general store at the Sesriem petrol station.
There is a bar at the Sesriem campground where most people hang out after a hard day of hiking in the desert.
There are a number of luxury resorts in the surrounding area, as well as a government-run campground with good facilities which is very close to Sossusvlei.
Don't get lost while wandering in the desert. And remember to drink plenty of water (as you should be doing throughout your time in Namibia, as it is a very dry country). Plan on taking at least 1.5 liters of water per person, even if you are a fit hiker. Big Daddy is approximately a 1-1.5 hour hike for moderately fit people, and then a roughly 2 minute run straight down, followed by a 20 minute walk back to the car area (though you will probably want to stop and take pictures in Deadvlei). If you plan on hiking a dune, count on at least spending 5 hours in the park including the drive to/from the Sesriem gate, so even if you arrive at sunrise, think about suncream!