The WDK was an attempt at combining the tools needed for developing drivers (formerly, the DDK) with the tools needed for logo-ing them (formerly, the HCTs). It might have sounded like a good idea at the time, but the result was a bloated 2.5GB download that almost nobody needed or wanted.
Thankfully it looks like we're moving back to the old model of keeping the two kits separate. Microsoft recently released a new kit called the Windows Logo Kit (WLK) that provides everything you need to logo your driver, with the plan of creating a new WDK that just contains stuff that was formerly in the DDK (build environment, headers, libs, samples).
Pero, don't get too excited, the days of the 2.5GB WDK download aren't over quite yet. Even though the WLK has been created and released on Microsoft Connect, the slimmed down WDK has yet to be released.