Mac OS X is based off of UNIX, which means that there isn't one, big, monolithic "registry" in which application and preference data is stored. Each program on Mac OS X uses its own preference file, keeping one application's settings completely separate from another's. If one preference file gets corrupted, all other applications' preferences will be safe, since they're in different files.
Preference files are stored in /Library/Preferences and ~/Library/Preferences, with some settings being stored in /Library/Application Support and ~/Library/Application Support. For example, Transmit (an FTP program by Panic) stores settings in ~/Library/Preferences under the filename "com.panic.transmit.plist" -- which translates roughly to the company "Panic" followed by the application "Transmit" followed by "plist", which is short for "preference list." Other applications follow a similar naming scheme for their preference files, too -- Apple keeps stuff under "com.apple.applicationname.plist", where "applicationname" is the name of the application for which the preferences apply.
Mac OS X also doesn't need to be "defragged" constantly like Windows does. The operating system takes care of a lot of housekeeping stuff automatically, and I recommend only performing manual maintenance routines when you suspect your system needs it, and not as a "ritual."
There also isn't one, big repository for browser histories and stuff. Safari can be "cleared" of history via the "Safari" menu (clear cache, reset all settings, etc.), and the same with Firefox... you'll have to clear each browser's histories using each individual browser, rather than locating one, central "history" holder.
