An octopus is a cephalopod. Cephalopods are the largest of mollusks. They are extremely fast and intelligent. Their eyes are very good at focusing in on prey. All cephalopods have either arms or tentacles. Octopuses have arms that are lined with suction cups for gripping and moving around. These suction cups are also commonly referred to as suckers.
Octopuses use their suckers to latch onto objects, the same way a suction cup does. These suckers can also move the objects around because they are made of special muscles.
The outer infundibulum is the muscle that controls the rim of the sucker. It changes shape so that it can create a seal around an object.
The inner acetabulum muscle forms the wall of the sucker. It contracts, causing negative pressure to build up. This creates the suction. The amount of grip a sucker has depends on how many of these muscles are flexed. More flexed muscles mean a tighter grip.
The extrinsic muscles allow the rim of the sucker to spin an object in a complete circle. This can be done at any angle to the arm without losing suction.