YouTube just added a new feature that allows you to add interactive commentary to your videos. Video Annotations(as a beta feature) let you add background information, create stories and link to related YouToube videos, channels or search results from within a video. Video Annotations are completely controllable, so you can control where/when the annotations appear and disappear or turn them off.
Adding annotations is simple. Once you logged in and started playing one of your video, you will see "Add Annotations" button on the right side of the player. Clicking on the button brings you to the editor where you can add annotations. After finishing adding annotations, just publish your video and preview.
Here's is an example video published with annotations.