The Phantom Zone is presented as a large, flat rhombus-shaped mirror that moves by spinning. It is used as an eternal prison, and is the destination for only the most heinous of criminals sentenced on Krypton.
Jor-El uses the Phantom Zone to imprison General Zod and his co-conspirators Non and Ursa, who appear to be transferred into the two-dimensional space on the mirror's surface, which is then flung into deep space. Superman's mother Lara Lor-Van refers to the Phantom Zone by name in Superman II when she first makes the revelation about the three villains contained inside it.
In Superman II, as Superman saves the city of Paris from destruction by hurling a nuclear bomb into space, the resulting nuclear explosion inadvertently shatters the Phantom Zone and releases the three prisoners. Now free, General Zod and his cohorts travel to Earth, wreaking havoc with the powers granted to them by Earth's yellow sun.
Supergirl is banished to the Phantom Zone by means of a summoned crystal shard by Selena. The crystal transports her to a barren, desolate world where it shatters, casting her to the ground. This depiction of the Phantom Zone suggests that the crystal shard seen in the first two Superman movies is not the Phantom Zone itself, but simply a vehicle that takes prisoners to this desolate wasteland which is referred to as the Phantom Zone. In this movie, it is also revealed that there is a way out of the Zone, but the trip to the exit portal is extremely dangerous, where Zaltar sacrifices his life to ensure Supergirl's escape.