I've been looking at the history of the USSR and while the secret police were fairly straightforward under the Tsarist regime and the USSR under Lenin (Okhrana and the Cheka), for Stalin's rule of the USSR, I've seen OGPU and the NKVD pop up concurrently.
I've established that both are secret police and used by Stalin as the instrument for the Great Purges and establishing terror throughout the country, but what is the exact difference?
I've asked someone and he has said that they are essentially the same, but surely there must be some distinction between the two, for they have two different names.
Thanks in advance.