Alix Lambert's fascinating documentary from 2000 about Russian criminal tattoos, The Mark of Caïn, is available under a Creative Commons license and viewable in full on YouTube. David Cronenberg, who used the film as reference for his own Eastern Promises, has said, "This is a very courageous documentary...I don't know how it ever got made, but it's beautiful, scary, and heartbreaking." From Wikipedia:
The Mark of Cain documents the fading art form and “language” of Russian criminal tattoos, formerly a forbidden topic in Russia. The now vanishing practice is seen as reflecting the transition of the broader Russian society. Filmed in some of Russia’s most notorious prisons, including the fabled White Swan, the interviews with prisoners, guards, and criminologists reveal the secret language of “The Zone” and “The Code of Thieves” (Vor v zakone).
The prisoners of the Stalinist Gulag, or "Zone," as it is called, developed a complex social structure (documented as early as the 1920s) that incorporated highly symbolic tattooing as a mark of rank. The existence of these inmates at prisons and forced labor camps was treated by the state as a deeply-kept secret. In the 1990s, Russia's prison population exploded, with overcrowding among the worst in the world. Some estimates suggest that in the last generation over thirty million of Russia's inmates have had tattoos even though the process is illegal inside Russian prisons.
The Mark of Cain examines every aspect of the tattooing, from the actual creation of the tattoo ink, interviews with the tattooers and soberly looks at the double-edged sword of prison tattoos. In many ways, they were needed to survive brutal Russian prisons, but mark the prisoner for life, which complicates any readmission to “normal” society they may have. Tattoos expressly identify what the convict has been convicted of, how many prisons he’s been in and what kind of criminal he is. Tattoos, essentially, tell you everything you need to know about that person without ever asking. Each tattoo represents a variety of things; cupolas on churches represent the number of convictions a convict has, epaulets tattooed on shoulders represent the rank of the individual in the crime world and so on and so forth.