Application installation and working is very different on Mac OS X than what it is on other operating systems such as Windows. On Mac OS X, installation usually involves just copying (or dragging and dropping) a *.app file from a DMG image file to your /Applications folder. Similarly, to uninstall an application, Apple recommends that you just drag the .app file to the Trash. Because of how this appears, many people think that a program is completely uninstalled (application files, settings, cache, properties, content) when it is dragged to the Trash.
However, this is not true. During the course of using an application, it might (and usually does) create other files on your system, most of which contain settings, properties and local content. These files reside in the /Library folder inside your home directory. While you may want to keep settings for some applications (such as a web browser), but for other applications which you are sure you will never use again, you might want to completely and cleanly remove all of its files. Its surprising, because this scenario is fairly common with most Mac users.
To facilitate this, and to save you the trouble of manually searching for all this files created by an app, you can use AppCleaner. AppCleaner is a freeware utility for Mac OS X, and helps keep your system free from unnecessary orphan files.
To uninstall an app, simply open AppCleaner, find the application(s) you want to uninstall in the Applications pane, tick them, and click ‘Search’. AppCleaner will look for all related files and display them to you, before confirming the deletion. Here you can see if any important file has been incorrectly flagged by AppCleaner as related to the selected application(s), and untick them if you do not wish to remove them. To delete the app and other files, press ‘Delete’.
If the application you want to remove is not in the /Applications folder, you can directly drag and drop the app into the AppCleaner window.
You can also delete widgets, plugins and install prefpanes from AppCleaner.