Launchpad is the application launcher that Apple has included in OS X since Lion was released in 2011. It makes it dead easy to find and launch applications on your Mac, regardless of where they're installed. Apps you've installed using the Mac App Store are really easy to remove - you just click and hold the icon, similar to what you'd do on the Springboard interface of an iOS device.
But apps you've downloaded elsewhere can be a bit trickier. Occasionally, the database that Launchpad uses can get corrupted, and you may find links to "dead" apps that are no longer installed, or missing apps that you know are on your Mac. Here's how to fix that problem: