Hrmmm... Don't see what's so hard about this. Took less than one minute search on Google (but too lazy? Everyone has to "spoon feed" you)

From Microsoft's Knowledge Base

A registry hive is a group of keys, subkeys, and values in the registry that has a set of supporting files containing backups of its data. The supporting files for all hives except HKEY_CURRENT_USER are in the Systemroot\System32\Config folder on Windows NT 4.0, Windows 2000, Windows XP, and Windows Server 2003; the supporting files for HKEY_CURRENT_USER are in the Systemroot\Profiles\Username folder. The file name extensions of the files in these folders, and, sometimes, a lack of an extension, indicate the type of data they contain.
Code:
Registry hive	                 Supporting files
HKEY_LOCAL_MACHINE\SAM	         Sam, Sam.log, Sam.sav
HKEY_LOCAL_MACHINE\Security	 Security, Security.log, Security.sav
HKEY_LOCAL_MACHINE\Software	 Software, Software.log, Software.sav
HKEY_LOCAL_MACHINE\System	 System, System.alt, System.log, System.sav
HKEY_CURRENT_CONFIG	         System, System.alt, System.log, System.sav,
                                 Ntuser.dat, Ntuser.dat.log
HKEY_USERS\DEFAULT	         Default, Default.log, Default.sav

Eh. I'll post it here. Too lazy to email it to you..