The biggest and baddest version of Windows out there, at least until Windows .Net Server rears its ugly vissage. Even though it's label as 'Windows 2000', it didn't get released until early 2001 -- but branding is important!
What is it, basically, is Windows 2000 designed for installations where scalability of initial design (or eventual expansion) is paramount, as well as reliability and raw data throughput.
Though it's very close in design and implementation to the others, Windows 2000 Professional, Windows 2000 Server and Windows 2000 Advanced Server, it does carry some important differences: can address 64GB of physical RAM (versus Advanced Server's 8GB), can use from 8 to 32 processors (but doesn't say if you can use less then 8 processors), does out-of-the-box 32-way load-balancing, hardware partioning (to run multiple installations on the same system concurrently) and has the added feature of 2- and 4- node clustering.
Clustering is probably the most important feature of W2KDS because it allows multiple nodes to behave as one logical node and can help avoid bottlenecks and administration issues common with load balancing distributed application across non-clustered machines. At least, that's the dream, to make multi-machine clusters behave as one true machine -- it works in theory so far, but not in practice.
Another change in DS is something Microsoft is calling Winsock Direct; this provides standard Socket level layer facilities that are optimized for high-speed and high-capacity SANs. Winsock Direct is an extension of Winsock that makes SAN networking as transparent as standard LAN-based networking, while providing the types of speeds expected from a LAN.
Don't ask how much W2KDS costs. Just don't bother; if you have to ask, you can't afford it. It's almost always offered with the package of a giant n-way server, and frequently that copy is specially tailored to that machine -- in fact, for DS, you have to get service packs and updates from the Hardware Vendor instead of from Microsoft. Though if you're really interesting in cost, know that it is sold on a per-CPU basis: 8-, 16- and 32-CPU packages are how the price is structured.