An architecture in Windows starting with Windows XP that stores OS components in related groups. Instead of updating and distributing a single DLL, the updated DLL is distributed along with all other DLLs related to the same function. If shared with all applications, the assemblies are stored in their own subfolder within the WinSxS folder. New versions of the assemblies are added to the WinSxS folder, but the older versions are not deleted. Private non-shared assemblies are stored in the application folders.
A peek into this WinSxS folder shows multiple versions of the same assembly. The arrow points to the DLLs contained in the highlighted subfolder. The Manifests subfolder contains XML documents that describe the versions, resources and dependencies of the assemblies.