WOW64(Windows-on-Windows 64-bit), which is the 32-bit x86 emulator that allows 32-bit Windows applications to run on 64-bit Windows.
The WoW64 subsystem is a lightweight translation layer that has similar interfaces on all 64-bit versions of Windows. Its primary purpose is to create a 32-bit environment that provides the interfaces required to allow 32-bit Windows applications to run unmodified in the 64-bit system. Technically, WOW64 is implemented using three dynamic-link libraries (DLLs): Wow64.dll, which is the core interface to the Windows NT kernel that translates between 32-bit and 64-bit calls, including pointer and call stack manipulations; Wow64win.dll, which provides the appropriate entry points for 32-bit applications; and Wow64cpu.dll, which takes care of switching the processor from 32-bit to 64-bit mode.
The WOW64 subsystem also handles other key aspects of running 32-bit applications. For example, it's involved in managing the interaction of 32-bit applications with the Windows registry, which is somewhat different in 64-bit versions of the OS, and in providing an interface to the storage subsystem.
The operating system uses the %SystemRoot%"system32 directory for its 64-bit library and executable files. This is done for backwards compatibility reasons as many legacy applications are hardcoded to use that path. When executing 32-bit applications, WOW64 redirects requests for DLLs from that directory to %SystemRoot%"SysWOW64, which contains legacy libraries and executables.