Here is how Webopedia explains it, which is as clear as anything I have seen:
"A database used by the Windows operating system (Windows 95 and NT ) to store configuration information. The Registry consists of the following major sections:
# HKEY_Classes_Root - file associations and OLE information
# HKEY_Current_User - all preferences set for current user
# HKEY_User - all the current user information for each user of the system
# HKEY_Local_Machine - settings for hardware, operating system, and installed applications
# HKEY_Current_Configuration - settings for the display and printers
# HKEY_Dyn_Data - performance data
Most Windows applications write data to the Registry, at least during installation. You can edit the Registry directly by using the Registry Editor (regedit.exe) provided with the operating system. However, you must take great care because errors in the Registry could disable your computer."http://www.webopedia.com/TERM/r/registry.html
Generally, if the key is a DUPLICATE, or points to a program no longer installed, it is comparatively safe to uninstall the keys. HOWEVER, be sure to use an application that will save the deleted entries, just in case you make a mistake and have to restore one or more of them.
Again, use caution; if you are unsure, then leave them alone. The space and time deleting them saves is not enough, in most cases, to compensate for possibly damaging the functionality of your computer.