There are many types of censorship, with the main categories being moral, military, political, religious and corporate. But many of us automatically associate the word "censorship" with unthinkable control such as by a Communist regime. Here is a photographic example of censorship in Russia in the year 1940::
Nikolai Yezhov, the young man strolling with Stalin was shot in 1940. He was edited out from a photo by Soviet censors. Such retouching was a common occurrence during Stalin's reign.
Sometimes censorship is absolutely called for at the workplace. Sometimes it's in the best interests of the organization as a whole for individuals to exercise self-restraint, which is a euphemism for censorship.
Why does this seem to be such a foreign concept? A little self-restraint of pen and tongue can make the world a better place. Whoops- that would be censorship, wouldn't it? Heaven forbid.