An Oblivion Gate is a portal designed by Daedric forces to create a gateway between Mundus and the realm of Oblivion. They were used during Molag Bal's attempted conquest of Nirn, the Planemeld, and Mehrunes Dagon's attack during the Oblivion Crisis.
The Oblivion gates act as bridges between the Mortal plane (Mundus) and the Daedric planes. Mehrunes Dagon used oblivion gates to lay siege to Tamriel, the gates were opened by a group of fanatic cult followers of Dagon called the Mythic Dawn.
As demonstrated during the Oblivion Crisis, a variant of Oblivion Gate called Great Gates can be created to deploy massive weapons, such as Siege Crawlers. These Great Gates are several times larger than standard gates and are used during large-scale attacks on cities.