It's easy to download YouTube videos—really easy. The simplest way is to use a web browser extension. Here are my picks for the major browsers:

• Safari: FastestTube extension (http://fastesttube.kwizzu.com/)—also works with every other browser too (including Internet Explorer) but I prefer others…
• Chrome: YouTube Options (https://chrome.google.com/extensions/detail/ojmgeoecaejeajjegjmijbcifhkbmgjd)
• Firefox: DownloadHelper (http://www.downloadhelper.net/)

That should cover it!