The difference between formal and informal censorship comes in the degree to which each is mandatory and is driven by formal rules.  Formal censorship has strict rules about what may and may not be published.  It is mandatory for the media to obey these rules, which have the force of law.  By contrast, informal censorship is not based on set rules and laws.  Instead, it is a much more voluntary thing.  In this system, the media voluntarily censors itself.  It does so perhaps because it feels pressured to do so by the government.  Or it may do so because it feels that cooperating with the government is the patriotic thing to do.  Either way, there are no set rules and there are no laws that require this type of censorship.