The 64-bit version of Windows is designed to make it possible for developers to use a single source-code base for their 32-bit and 64-bit Windows applications. To a large extent, this is also true for 32-bit and 64-bit Windows drivers.
For user-mode applications, 64-bit Windows includes a Windows on Windows (WOW64) thunking layer that enables 32-bit applications to execute (with some performance degradation) on 64-bit versions of Windows. It does this by intercepting 32-bit function calls and converting pointer-precision parameter types to fixed-precision types as appropriate before making the transition to the 64-bit kernel. This conversion process is called thunking.
Note This thunking is only done for 32-bit applications; 32-bit drivers are not supported on 64-bit versions of Windows.