Classified information is material that a government body deems to be
sensitive information that must be protected. Access is restricted by law or
regulation to particular groups of people with the necessary security
clearance and need to know. Mishandling of the material can incur criminal
penalties.
A formal security clearance is required to view or handle classified material.
The clearance process requires a satisfactory background investigation.
Documents and other information must be properly marked "by the author" with
one of several (hierarchical) levels of sensitivity—e.g. restricted,
confidential, secret, and top secret.