Video Annotations is a new way for you to add interactive commentary to your videos! Use it to:

• Add background information about the video
• Create stories with multiple possibilities (viewers click to choose the next scene)
• Link to related YouTube videos, channels, or search results from within a video
• All of the above!

You control what the annotations say, where they appear on the video, and when they appear and disappear.

Ref:

http://www.youtube.com/t/annotations_about

https://support.google.com/youtube/answer/92710?hl=en