There are several types of overlays in the computer world. One type is a video overlay, which is a technique to display video on a computer monitor without having to go through the computer processor. This type of overlay is commonly used by TV tuner cards and early 3D graphics cards.
Another type of overlay is a hardware overlay, which is also a type of video overlay. This type of overlay uses dedicated memory inside the video card to render an image to a computer monitor. This process helps to improve the video quality for a computer game, a DVD or a TV tuner card. Most video cards since 1998 support hardware overlay.
A third type of overlay is an overlay network, which is basically a computer network on top of another network. This type of overlay is used with private networks to improve routing. It is also used on the Internet for routing data to a destination that are not specified by an IP address. This would normally not be possible without an overlay network.