In a nutshell, you paste the URL from a video that you want to download and then you have some options. You can just download the audio as an mp3 in various levels of quality or you can do the same with video. Then press the Record button and you're done. I tried a YouTube, Vimeo, and TeacherTube video and they were all recognized by the system. In fact, Dirpy.com did a good job of grabbing some videos that weren't available using other tools.
Best of all Dirpy.com is free and has no advertisements! There is also a bookmarklet you can drag into your browser's bookmarks bar and when you come across a YouTube video you want simply press the bookmarklet and it takes you to Dirpy.com and has the video ready to download. Great tool!