There are a couple ways to incorporate a You Tube video into a PowerPoint presentation - you can link out to the Web to view it, or you can embed it within a presentation slide. Linking out is nice because your overall file size is smaller, and therefore more portable. However, having it play within the presentation itself is nice because it keeps you within the presentation, and not having to go back and forth to different windows while you are presenting.
Learn how to embed a You Tube video into a PowerPoint presentation.
Learn how to link out to a You Tube video from a PowerPoint presentation.