Your Mac is full of files: big ones, small ones; documents, music files, photos, videos, and system files. A quick check of my iMac’s 256 GB SSD shows that it contains more than 60 million files. You can see this number on your Mac by opening Disk Utility (which is in your /Applications/Utilities folder), selecting your drive, and then clicking the Info button in the Disk Utility toolbar–then scroll down to the File Count entry.
Naturally, you haven’t created all those files. Most of them are system files, preference files, bits of applications (an OS X application is actually a bundle, or a sort of folder containing thousands of files), and more. Most of the files you’ve created are in your Documents folder, or in other media-specific folders inside your home folder: Music, Pictures, or Movies. And some of your personal files may actually be in your Library folder; this includes your email messages and their attachments, for example.
Among these millions of files, your Mac may contain many duplicate files, and there are two types of duplicates: intentional duplicates and accidental duplicates.
Read the rest of the article on The Mac Security Blog.