Censorship is when an authority (such as a government or religion) or other group cuts out or suppresses communication.
This has been done widely. All countries, religions and societies have their limits as to what can be said, or written or communication by art or nowadays by computer.
Certain facts are changed or removed on purpose. This may be done because it is considered wrong, harmful, sensitive, or inconvenient to the government or other authority. This can be done for different reasons.
A censor is a person whose job is to look at all types of media and remove material. There are many reasons to censor something, like protecting military secrets, stopping immoral or anti-religious works, or keeping political power. Censorship is almost always used as an insult, and there is much debate over what censorship is and when it is okay.
When there is freedom of speech and freedom of the press, most information can published. However, even in developed countries with much freedom of the press, there are some things that cannot be published. For example, journalists are usually not allowed to publish many secrets about the military, like where troops will be sent on a mission. Pornography is censored in some countries because it is seen as not moral. For these reasons, the government might arrest anyone who publishes it.