Camtasiaa initiated the technology to insert descriptions.
Now available on most editing software, including windows movie maker.
Youtube have recently realized the importance of adding informative text for members to display on their uploaded videos.
Now on youtube, when you open 'my videos' (your videos), you will notice a new box - 'ANNOTATIONS', click on this to edit, where you can either enter a speech bubble or 'add note' - which will place your message into an adjustable box.
If you simply want to add a website address, (example video below),
you must adjust the timer to display from beginning to end.