c0c is sort of a code of conduct designed to focus on programming instead of political and social aspects. While we still expect everybody to be nice and behave reasonably, the c0c makes it clear that people are judged by their contribution to the project, and not by gender, political, religious or social beliefs.
This is the original c0c document (written in plain html). There is also a rationale that explains more details and why specific points are included.
The c0c is also available in:
Normally it doesn't - often common sense just works.
An emerging trend in open source development is declaring CoC, "Code of Conduct" for projects. The generic idea of making rules explicit is an useful one. However the specific rules dictated by a recent wave of CoC documents may be suboptimal.
These documents (e.g. this one) focus on social aspects instead of coding aspects - often ignoring the fact that the group is formed primarily in order to code. These documents deliberately and aggressively shift the focus from the project's subject matter to political qustions and gender questions.
Not every project is happy with that. Some projects prefer to go with rules focused on the act of programming and collaboration instead of social aspects. c0c is a simple rule set for projects where developers want to keep the discussion focused on technical aspects.
It is sort of a reaction: many projects implicitly applied these rules for decades, as common sense, without having to codify them. The recent CoC-trend made some users push for making the rules explicit.
Not really - the c0c is not about being mean to each other. We still expect people be polite, and projects should ban those who are not constructive members of the community but join mainly to bully other people.
The difference is that c0c makes it clear that the focus is on programming aspects, and not gender/politics/diversity. Which does not mean we treat any group of people differently. Quite the opposite: we explicitly do not care about your gender, any property of your body, but only about what you do within the project.
The author of this c0c is a long time open source programmer, Tibor 'Igor2' Palinkas. Contact him via email: c0c [at] igor2.repo.hu