Somewhere along the way, you'll come across an unfamiliar file you don't recall downloading (or even exactly what it does). In this age of identity theft (which, admittedly is almost impossible when using the latest Mac OS X), you can click on the file's icon, then press Command - I which opens up a dialog box. Click on the right-facing arrow beside "More Info" and magically, it expands to show exactly what the source of the file is, along with the last date you opened it.