Sometimes when you try to empty the trash in Mac OS X, you will receive a message saying that one or more items that are in the trash are locked or in use and cannot be removed. You can choose to continue anyway, but those items will remain in the trash, as they could not be deleted.
The first thing you should do, of course, is to check that actually those items are not in use. Sometimes you may have a document open in Word or some other program and later on you decide to trash the document using the Finder. Of course, the document is still open in Word so the Finder won't allow you to delete it. Close the document and empty the trash again and everything should be fine.
However, sometimes there are files in the trash that are simply locked or the Finder thinks they are in use, when in fact they are not. You don't really care, you want them GONE. In this case, you can use a neat little trick to force the Finder to empty the trash and delete those items, even if they are locked or in use. You simply hold down the Option Key on your keyboard while you choose the Empty Trash command from the Finder menu. If you look closely, when you are holding the Option key and you click the Finder menu, the Empty Trash command no longer has the three dots '...' after it. This indicates that you will not be prompted during this action. The trash will be emptied and you will not receive any warnings. Locked files or those that are in use will be removed.
In case you don't know which key is the Option Key (sometimes also labelled ALT), the following image will show you where to find it on your keyboard: