A fetus stalk
An adult pod
A pod is a receptacle where synthetically created humans are inserted and hardwired into becoming part of the power plant and virtually integrated into the Matrix.
Humans are grown in the Fetus fields and, based on Morpheus' description of pods, are immediately connected by myriad umbilicals that not only provide the body with sustenance, but draw heat energy from the body to power the Machines and their city. They are gathered and transferred from by Harvesters to full pods in the power plant where several more plugs are added to the body. Each human is not only naked but also completely hairless, with the fluid in the pods preventing growth.
Central to the control of a human in the pod is a data probe that is firmly and permanently attached to the headjack, which provides connectivity of the human's mind into the Matrix. Each human pod can be compared to a radio station; a carrier signal for each human mind transmits and receives data from the Source to process information needed to form the neural-interactive world of the Matrix.
The human is part of the Matrix computer network until their body can no longer function, or if their carrier signal is interrupted, forcing the human to awaken. From there, docbots appear to be commanded to eject adult humans from their pod and into the cold, lifeless world of the sewers of the ancient remains of the human cities, presumably to their death.
Since a pod-human has never used their body in any significant manner, their muscles are very weak to the point of atrophy, so swimming is a near-impossibility. Young children and adults ejected from their pods may have been "helped" through the intervention of Zion operatives nearby, who rescue ejected humans with their hovercraft and nurse them to health to become part of the resistance against the Machines.
In their only conversation, the Architect warned Neo that, without his cooperation in reloading the Matrix, all of humanity, including the humans connected to the Matrix, would die if the Matrix suffered a system crash from complications caused by its programming.
Pods in a power plant shortly after the surrender of the United Nations at the end of the Man-Machine war. Unlike future power-plants, these pods were filled with people who were born naturally but were captured and converted into batteries by the machines.
Billions of these pods are connected to cylindrical towers that form the power plant. There are also very small football sized and shaped baby pods connected to 5ft tall bioelectrical stalks that can contain up to 5 or 6 fetuses at a time.