Conceptually, plug-ins are like Lego toys. They are software modules that add a specific feature or service to a larger system. A Lego wheel by itself isn’t that fun, but add that wheel to a Lego car, and you’re cooking with gas. Unfortunately, most plug-ins, whether for browsers or graphics programs, don’t provide as much functionality as a Lego wheel. For example, there are number of plug-ins for the Netscape Navigator browser that enable it to display different types of audio or video messages based on MIME types. However, if nobody develops those kinds of files, the plug-in is useless. There are some cool plug-ins for graphics programs, the best of which is Kai’s Power Tools. That adds a zillion effects that you can use on your images. By the way, in Photoshop you can activate plug-ins by holding down the command and shift keys when starting up the program.