Registry Files are files used by the Windows Registry. A collection of databases, the Windows Registry contains configuration settings, options, and other necessary details about program and hardware installed on Microsoft's Windows operating system — specifically low-level settings. It can be used by the kernel as well as device drivers, services, the user interface, and the Security Accounts Manager.
On your computer's disk, the Windows Registry is saved in a number of discrete files called hives, each of which can contains a Registry tree. These files are known as Registry Files and are created and managed by the system in the memory. There are a number of different file extensions used by Registry Files for different tasks and purposes, from “.REG” files used to update the Windows Registry to “.LOG2” files to store trace information.