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defaults is a command line utility in Mac OS X that manipulates plist files. Mac OS X stores each user's preferences set in a .plist file for each program stored at ~/Library/Preferences for user-specific preferences, and /Library/Preferences/ for global preferences. Either way, the defaults utility writes or reads from these plist files depending on the one given.

The most common uses of defaults is:

$ defaults read $PLIST_FILE # gets all
$ defaults read $PLIST_FILE $PROPERTY_NAME # gets
$ defaults write $PLIST_FILE $PROPERTY_NAME $VALUE # sets
$ defaults delete $PLIST_FILE $PROPERTY_NAME # resets a property
$ defaults delete $PLIST_FILE # resets preferences

$PLIST_FILE is replaced by the plist file, without the extension. plist files are named similar to Java packages: you take the domain name of the programmer/company, reverse it, place a dot, and then the program's name followed by .plist. So for example, iTunes' plist file would be com.apple.iTunes.plist. When you pass the value to defaults, you remove the .plist:

$ defaults read com.apple.iTunes # prints all of iTunes' preference values

plist files store property lists and values. The $PROPERTY_NAME value becomes the name of the property to modify. For example, to remove the Search field from Safari's address bar:

$ defaults write com.apple.Safari AddressBarIncludesGoogle 0

or

$ defaults write com.apple.Safari AddressBarIncludesGoogle -boolean FALSE # case-sensitive!

Replacing 0 with 1 or FALSE with TRUE returns the search bar to normal.

Sometimes, preferences cause corrupt programs. To reset Address Book's preferences, you would either remove ~/Library/Preferences/com.apple.AddressBook or issue the command

$ defaults delete com.apple.AddressBook