Extravehicular Activity Pods (often abrieviated simply as "EVA Pod") are small transports designed to move outside of a spaceship, often to provide maintenance.
The American spacecraft, Discovery One, carried three such pods (named Alice, Betty & Clara). Two were lost during the mission to Jupiter. The first was tossed deeper into space after being hijacked by the psychotic computer, HAL 9000. HAL used it to murder Discovery's deputy commander, Dr. Frank Poole, who was outside the pod. The second was lost along with mission commander David Bowman, when he disappeared into the very large Monolith above Jupiter. The remaining Discovery pod had a missing egress door, blown off in Bowman's attempt to reenter through the emergency airlock after the on-board HAL computer's refusal to let Dave enter the pod bay. Nine years later, Bowman's pod was found by Discovery designer Walter Curnow & cosmonaut Maxim Brajlovsky, American & Russian crew members of the investigating Soviet spacecraft Alexei Leonov. They re-entered Discovery One - now covered in sulfur from the volcanic activity originating on the moon Io - as it orbited Jupiter.
Normally, the pod that Dave used to enter the ship would have sailed off into space due to the same escaping air pressure that blew out the pod's door. However it is possible that Dave returned the pod to the ship by remote, or that it was attached by one of its manipulator arms.
The EVA pods housed by Discovery were completely self-sufficient vehicles. They had their own maneuvering and life-support systems, and the interior could be pressurized and kept at a completely normal shirt-sleeve environment, so that one could perform an EVA in the pod without having to wear a full spacesuit.