Steel Sheet is any metal that is thicker than a foil and thinner than 6mm, the thickness of a metal plate. Steel Sheet is often used for building structures that do not require durability. It is also often corrugated or diamonded for additional strength without increasing weight. Corrugation is the creasing of the metal at regular intervals to form ridges, and diamonding is the addition of diamond ridges that add structure to the metal.
Once the steel is more than 6mm thick it is known as steel plate. Steel Plate is used in applications where durability is more important than saving weight. It is used in automobiles where durability is required to pass crash testing.
What's the difference
The only difference between sheet and plate steel is the gauge (thickness) of the metal. They both have very different uses, depending on the varying durability and weight requirements for different projects.