There are many, many "defaults" spread out across the entire system. There is no one, master command you can execute to reset everything back to it's default settings.
Most applications store their preferences in .plist files, usually located in either /Library/Preferences or ~/Library/Preferences. If you wish to reset an application to its default preference settings, you would simply remove the corresponding .plist file from one of those directories.
What is the exact problem you're having?