When you drop a file into the trash can and hit erase, it really isn't gone. OS X simply marks the file as gone, and the data is still on the hard drive, until it is overwritten with new data. This is the reason that some programs are able to 'un-erase' files. If you are dealing with potentially sensitive data, and you want to erase it, use a built in feature in Mac OS X 10.3 and higher.
If you want the file to really be gone, you can use the 'Secure Empty Trash' from the Finder menu. This erases the files in the trash, and then writes several times over the data with gibberish. This makes the file nearly impossible to recover. I say nearly because emerging technologies could make this less secure. For now, it meets the Department of Defense standards, and I feel pretty safe with it.
One thing to consider.. if your program (such as MS WORD) writes temporary files.. these could still be on the machine, so that's always a good thing to check. If you want some more info on the subject, and other Mac Security Features, check out this PDF document . Download