I don't know the exact term, but I'm wondering what its called when their is a rule or law that is not written, but it is universally observed. People obey this rule or law even if its not written. What are these type of rules or law called?
They're called customs. There's "customary law," which recognizes the way things are done traditionally if there's no written law.
Customs come into play in particular with "customary international law." For many international issues, they simply need to look at the way things have been done by custom in the past. They don't rely so heavily on some fixed international set of laws, since it would be virtually impossible to maintain.