Video sitemaps are an excellent way to make sure that Google knows about all the video content on your site, especially content that we might not otherwise discover with our usual crawling mechanisms. Improving the findability of your video content can improve your site's appearance in Google Video Search results.
When you submit a sitemap that has video information to Google, Google makes the included video URLs searchable on Google Video. Search results contain a thumbnail image (provided by you or auto-generated by Google) of your video content, as well as the information (such as a title) contained in your sitemap. Note that we can't guarantee when or if your videos can be added to our index as Google relies on complex algorithms for this process.
You can create a separate sitemap listing your video content, or you can add information about your video content to an existing sitemap—whichever is more convenient for you. The Google video extension of the sitemap protocol enables you to give Google descriptive information—such as a video title, description, duration, etc.—that makes it easier for users to find a particular piece of content.
Note, it is possible that Google might instead use text available on your video's web page rather than the text you supply in your sitemap.
Video tag definitions
Read the full syntax and details of video sitemaps here.