A secret police (sometimes political police) force is a police organization that operates in secret to enforce state security. This blanket term generally means keeping the government from being attacked from within (e.g. sabotage, revolution, etc). In countries where rule is by fiat the secret police are often used to do things that the rulers cannot be seen to do openly.
In some countries, such as police states, dictatorships and totalitarian states, the secret police often uses methods that are or would usually be considered illegal (violence, killings, blackmailing, intimidation, disappearances) to suppress sedition, dissent, or political opposition.
This can also happen in states which are usually described as "democratic". For example, the United Kingdom's treatment of the Irish before Ireland achieved independence and since has included shooting people down in the street and intimidating people in the middle of the night.
There are, of course, different varieties of democracy and, in times of emergency or war, a democracy can grant its policing and security services extra powers. These emergency situations can be abused or even manufactured. According to The Rise and Fall of the Third Reich by William L. Shirer and other research and writings the Nazis in Germany of the 1930s manufactured an emergency by burning the Reichstag and then blaming it on the Communists.
Which entities can be classed or characterised (in whole or part) as a secret police organisations is hotly disputed, with, for instance, one side including the CIA and MI5 under the heading of "secret police" as the other maintains that organisations that are essentially for foreign intelligence-gathering and monitoring are not thus "police" and should not be so called. Another controversy is over whether the FBI and United States Secret Service must be included because secret-police activities such as wiretaps and what they characterise as "home invasions" are sanctioned, while the other side of the argument argues that such organizations do not engage in the repression, torture, and summary executions characteristic of other "secret police" organizations. A major issue of the argument is whether the term "secret police" connotes repression or rather the extensive use of low-visibility tactics. The biggest allegations that the FBI constituted a secret police relate to the Vietnam era, when the organization infiltrated and attempted to subvert political organizations deemed dangerous under the directive of the COINTELPRO. Recently, the Human Rights Watch organization has accused the CIA of "disappearing" al-Qaeda prisoners.
The concept of secret police is also popular in fiction, usually portraying such an institution at its most extreme. Perhaps the most famous example is the Thought Police from George Orwell's famous novel Nineteen Eighty-four. In that world, the Thought Police used psychology and omnipresence of surveillance to find and troubleshoot members of society who have the mere thought of challenging ruling authority. Real secret police are not, of course, this powerful, or at least not according to reports, though reports can be tampered.