Okhranka (Okhrana) is the secret police of late Russian Empire. The full name is Security Bureau (Okhrannoje Biuro, in Russian) of the Ministry of the Interior. It was informally called "Okhrana". "Okhranka", or "tsarist okhranka" is its derogatory naming, used by revolutionaries and other people dissatisfied with tsarist way of rule.
As the name suggests, its primary purpose was the state security, i.e., struggle against hostile organizations: plots, terrorists ("bombists"), socialists, and revolutionaties.
The task was performed by all means, including covert operations, undercover agents, "perlustration" -- reading of private correspondence. Even its Foreign Agency served this purpose. The Okhranka is notoriously known of its agents provocateurss Azef, Bogrov, the Bloody Sunday event, and fabrication of the antisemitic Beilis trial.
References
Charles A. Ruud, Sergei A. Stepanov; Fontanka 16 - The Tsars' Secret Police; McGill-Queen's University Press (paperback, 2002) ISBN 0773524843
