Classified information is information which has been deemed sensitive enough that access to it is restricted. A classic example of this type of information is military intelligence, which is circulated only among the people who absolutely need to see it to reduce the risk of potentially catastrophic leaks of information. All governments and many large organizations such as corporations have systems in place for identifying and securing classified information to ensure that it does not fall into the wrong hands.
The concept of classified information is quite ancient. The Ancient Greeks and Romans both had systems in place for securing information, and some societies had traditions of using deaf servants at confidential meetings to ensure that guests were well provided for while the information at the meeting remained secret. With the development of increasingly complex technology, classified data has become correspondingly more complicated, as have access restrictions.
Most governments differentiate their classified information into several levels, ranging from top secret information which is only seen by a handful of people to unclassified information, which is open to the general public. People who work for the government receive a security clearance which details the information that they can access; security clearances are typically built into accounts on government computer systems and id cards so that only people with the right clearance can access highly classified data.
When information is evaluated to determine whether or not it should be classified, the primary concern is national security, or in the case of an organization, the security of that organization. For example, the fact that a President is visiting a particular city is not considered classified information, but the details of his or her route would be classified, because someone could potentially use this information to attack the President.
As long as humans have been classifying information, other humans have been trying to access that information. There have been several famous historical incidences of people passing on classified organization from within organizations, or penetrating organizations with the purpose of gaining access to classified information. Willful transmission of classified information to the enemy is generally considered treason.
Periodically, a government will declassify historical documents so that historians and other people can examine them. The Central Intelligence Agency (CIA) in the United States, for example, has a large archive of declassified documents, some of which are quite interesting, like the files of high-profile individuals from previous historical eras.