In-stream ads play like a TV-style ad before or during another video from a YouTube partner. Viewers see 5 seconds of your video and then can keep watching or skip it.
In-slate ads show before YouTube partner videos that are 10 minutes or longer. Viewers choose to watch one of three ads or see regular commercial breaks during their video instead.
In-search ads appear above or to the right of regular result on the search result page.
In-display ads appear alongside other YouTube videos, or on website on the Google Display Network that match your target audience.