Operating systems can sometimes be very vague when it comes to identifying used and free space on a computer. If you've ever seen the "other" category taking up a large percentage of space on your hard drive, then this should help clear that up.
A good third party app for Mac is Disk Inventory X. If you're using Windows, you can download WinDirStat, which stands for "Windows Directory Statistics." Both of these programs give a detailed, graphical illustration of used space on your PC or Mac. There are big colorful blocks that represent used data, and clicking on the blocks will tell you what is using that space.