In-stream refers to the placement of the ad. An in-stream ad is an online ad that plays in between content that you are watching on the web.
Gateway, also known as pre-roll ads, can be in three different formats: MP4, Flash, and MP3. Flash pre-rolls are limited to desktop players only.
A Gateway Ad plays before a video, listen to a podcast, or listen to a music stream. A gateway ad can be a video ad or an audio ad, and either type can have an optional companion banner displayed while the gateway ad is played.
The last ad type is the display banner ad. Display banner ads are a rectangular ad, that display at the bottom of the player.
In-Stream ads and gateway ads can also have a companion banner ad. A companion banner ad shows simultaneously with the in-stream or gateway ad.
For instance, if you are streaming music and an in-stream ad comes on, a companion banner ad could also display on the screen at the same time.
•Creating a New In-Stream or Gateway Ad