You can't copy them in the traditional sense, but you can create an account and save videos to playlists to watch later. You can also copy links and/or code to post them on other sites or email them.
Here's a screenshot of a YouTube vid to help you understand...
See the "thumbs up" symbol in the photo above? When you want to get the link to a YouTube video, click the last symbol in the row. The one to the right of the + sign. It will give you options for what type of link you need.
Good Luck!