Skippable Ad on YouTube appears before the beginning, in the middle of, or in the end of a YouTube video. This ad can be skipped by clicking the 'Skip ad' button in the right corner at the bottom of a video.  

YouTube Display ad appears on the top of all the videos similar to the niche one is searching for on YouTube. YouTube calls it “In-feed Video Ad,” as it appears in the feed section. YouTube display ad is thought to be one of the best ad type on YouTube to bring effective and strong engagement to content.