According to the 1946 edition of the US Navy Bluejackets' Manual:

"Any information or material which must be circulated among naval and military personnel, but which must also be safeguarded against leakage to the enemy, is classified and marked in one of three ways.  The classification of documents determines how many people will see them.

Secret. - Secret matter is the most closely guarded and is considered to be of such importance that disclosure might endanger the national security.  One subdivision of secret is top secret.  Documents so classified are seen by very few people indeed.  Any knowledge of secret matter which comes to a man outside of his assigned duties should be reported immediately to his superiors so that the leak may be found and plugged.

Confidential. - Confidential matter is second in importance and is of such a nature that disclosure would be harmful to the interests or prestige of the nation.  Confidential material is never left unguarded.  Any breaks in the security of confidential matter should be reported immediately.

Restricted. - This is the lowest classification.  Restricted matter is limited in circulation to government or military personnel who are properly concerned with the information.  Often matter which would normally be classified as confidential is classified restricted because the wider circulation is of more benefit than the additional security."

After the end of the war most Restricted material was declassified and most Confidential material was downgraded to Restricted.  In the mid-50's the remaining Restricted material was declassified.  Declassification of Secret material was handled on a case by case basis, with many items still classified over 60 years later.