In the strict sense, censorship is an act of government in which it becomes criminal to obtain or disseminate certain types of information.
The purpose of censorship is to control people by influencing the way they think and act. It is understood that people's thoughts and actions are shaped by the information they have available. To the extent one can control what information people have, one is able to control the people themselves. For this reason, censorship is very common among, perhaps even essential to, totalitarian governments.
In a less strict sense, censorship means any attempt to prevent people from obtaining or disseminating information. This would include, for example, a newspaper that refuses to run an advertisement it considers inappropriate, or a lecture hall that refuses to rent itself out to a particular speaker. This sense of the term is often considered incorrect, in that it implies the 'censored' party has some right to use the property of the 'censoring' party.
We need more than just the meaning and purpose of "censorship"! How about:
Censorship in the United States (books, art, etc. censored)
Censorship in Europe (books, art, etc. censored)