Files deleted on OS X aren’t gone forever (at least not initially). They get put into a special place on the system until you are really sure you want to get rid of it. This special place is called, imaginatively enough, the Trash. The video below walks you through how the Trash can works
Files end up in the Trash a number of different ways, you can select delete from the Finder menu, you can drag files into the Trash. Also, some Mac programs support the Trash so for example, when you delete files from iTunes, they first go into the Trash.
The beauty of having a Trash Can is that if you inadvertently delete a file, you can easily recover the file. In order to do this, you just click the Trash can, choose the file from the Finder window and pull it out of the Trash.
Eventually, however, you may want to actually take the trash out. The main reason for this is that you are eventually going to want to reclaim the space needed by the files in the Trash.
There are a few ways to delete files from the Trash. The easiest is to select the Finder menu and click the Empty Trash menu item. You can also do this with the Cmd+Shift+Delete key combination. Finally, you can right click on the Trash icon on the dock and choose Empty Trash. If you don’t have a two button mouse press the Ctrl key while clicking the Trash.
Doing any of this will remove the files from the Trash can. For all intents and purposes, these files are deleted from your system. However, a clever person could figure out a way to retrieve these files. If you have anything that is particularly sensitives and have a laptop, you may want to use the secure delete feature. Think of secure delete a a document shredder. When you secure delete a file a hacker wouldn’t be able to retrieve the contents of the files. To secure delete your files, select the Finder menu and click the Secure Empty Trash menu item.
If you wish, you can enable Secure delete by default, doing so will ensure that your documents are shredded everytime you empty the Trash. The downside to doing this is it takes longer to delete the Trash.
Enabling Secure Delete by default is easy. Click the Finder menu and select the Preferences menu item. Select the Advanced icon and check the Empty Trash Securely checkbox.
You can place an item in the Trash that you are currently using. This is fine until you go and empty the Trash. If that happens, OS X will display a dialog box explaining that the file is in use. You can solved this problem by closing whatever program happens to be using the file and try emptying the Trash again.