Pictured: Classified
In the U.S., information is called "classified" if it has been assigned one of the three levels: Confidential, Secret, or Top Secret. Information that is not so labeled is called "Unclassified information".
The desired degree of secrecy about information is known as its sensitivity. Sensitivity is based upon a calculation of the damage to national security that the release of the information would cause. The United States has three levels of classification: Confidential, Secret, and Top Secret. Each level of classification indicates an increasing degree of sensitivity. Thus, if one holds a Top Secret security clearance, one is allowed to handle information up to the level of Top Secret, including Secret and Confidential information. If one holds a Secret clearance, one may not then handle Top Secret information, but may handle Secret and Confidential classified information.
Levels of classification used by the U.S. government
Core secrets
The highest level of classification. Information at this level is released only to select government individuals (NSA exclusively).
Top Secret
The highest security level outside of the NSA framework. "Top Secret shall be applied to information, the unauthorized disclosure of which reasonably could be expected to cause exceptionally grave damage to the national security that the original classification authority is able to identify or describe." It is believed that 1.4 million Americans have top secret clearances.
Secret
This is the second-highest classification. Information is classified Secret when its unauthorized disclosure would cause "serious damage" to national security. Most information that is classified is held at the secret sensitivity.
Confidential
This is the lowest classification level of information obtained by the government. It is defined as information that would "damage" national security if publicly disclosed without the proper authorization.
Public Trust
Despite common misconception, a public trust position is not a security clearance, and is not the same as the confidential clearance. Certain positions which require access to sensitive information, but not information which is classified, must obtain this designation through a background check. Public Trust Positions can either be moderate-risk or high-risk.
Unclassified
Unclassified is not technically a classification; this is the default and refers to information that can be released to individuals without a clearance.