regsvr32.exe

This is an abbreviation for Register Server.
This is a command line utility that is used to register unmanaged OLE controls such as dlls and ActiveX controls

When this utility is used the DllRegisterServer and DllUnregisterServer methods will be called.
This utility is not concerned about what these methods do, it just checks runs them and returns a value.

32 bit window - C:\Windows\System32\regsvr32.exe
64 bit window - C:\Windows\SysWoW64\regsvr32.exe

Command line Options

Regsvr32 [/u][/n][/i[;cmdline]] <dllname>

/u - unregister dll
/n - do not call DllRegisterServer (must be used with /i)
/i - call DllInstall passing an optional [cmdline]. When used with /u it calls DllUninstall
/s - silent (no message boxes are displayed)

Registering a File

regsvr32 myBetterFile.dll 

UnRegistering a File

regsvr32 /u myBetterFile.dll