Perhaps a good way to define secret police is that its a government run organization which regularly performs under-the-table and outside the law actions against supposed internal threats to the government. In a sense, no better than a hit squad that just happens to be run by the ruling government.
Perhaps the most famous example of a secret police force in history is the Russian KGB (Komitet Gosudarstvennoy Bezopasnosti, literal translation "Committee for State Security"), the Soviet Union's intelligence and secret service agency, which is as of now disbanded.
Although it was a publically known organization like the Secret Service, the KGB has a great deal of mythos and rumor surrounding its existence. The organization was notorious for having a plethora of supposed cloak-and-dagger dealings both internally and internationally, which depending on one's point of view may show the KGB's secret police qualities despite being a law-bound government organization.
The very definition of secret police is really very speculative in nature. Depending on one's point of view, the United States CIA and even the FBI could exhibit unofficial, secret police like qualities. Maybe its a matter of just how much unwarranted action is taken by an organization. In other words, just how brutal, lawless, and unfounded the methods are.