Things that are strictly prohibited are:
- Technical abuse: spam, flood, (D)DoS etc.
- Things that are explicitly illegal in EU, USA or Canada.
Also try to not be a complete asshole. There are no rules against that, but that doesn't mean you have to be one.
There are no network-wide rules all channels and their participants have to follow beyond the ones above. Channel ops can additionally declare any other rules local to their channels and enforce them (or not) however they want.