The Okhrana were from, 1881 to the Russian October Revolution of 1917, the feared secret police of Czarist Russia. They were founded by Alexander III, as part of a generally reactionary domestic policy, after his accession to throne and the assasination of his father, Alexander II, by revolutionaries. Their main task was the suppresion of political and labor dissent, and they were probably responsible for around 26,000 deaths, both in Russia and abroad.
The Okhrana's favorite method of operating was through co-opting people. They controlled a vast network of informers, double agents, agents provocateurs and front groups, operating out of both St. Petersburg and Moscow, as well as foreign cities with large expatriate Russian populations, particularly Paris and London.
The main enemies they were commissioned to fight were communist, socialist and anarchist groups, as well as trade unions. For the most part they were tremendously successful, in that they managed to put infiltrators in place at every level of these groups, including a huge percentage of their leadership. Their reputation was even more fearsome, and they probably accomplished almost as much from the constant terror that their name and reputation caused across all of Europe as their actual actions.
The Okhrana were probably directly responsible for the promulgation of the Protocols of the Elders of Zion hoax, as a move in the byzantine domestic politics of Russia. Some conspiracy theorists also implicate them in the Jack the Ripper murders.
Ironically, after the Bolsheviks took over in Russia, they took control of the Okhrana's files, which they were more than happy to use for their ends. They were so impressed with the quality of the Okhrana's work that they started their own secret police, the Cheka, along the same lines and using the same techniques. The Cheka would later become the NKVD, and then the famous KGB. There is even some speculation that Stalin may have started his career as an Okhrana informant.