Linear Video Ads
These are ads that we commonly call pre-, mid- and post-roll ads. The key to a linear ad is that it takes over the full video player space. This type of ad can also include an interactive takeover, for example an overlay that gives a user a choice of which ad to watch or when to watch the ad. They're linear because they run in line sequentially with the content as in ad-video, video-ad-video or video-ad.
Linear ads are, again according to the IAB, either 15- or 30-second spots. They can be less but should not exceed 30 seconds. Do not allow for fast forwarding through the ad. The ads can be 4:3 or 16:9 aspect ratio, however, 16:9 is far more common nowadays for online video players.