Well, perhaps it's best to start with what it's not.

It's not a single, official big book of laws, and it's not enforced by an international police.

And cases are not heard by a single court.

No, it's a little more complicated than this.

Instead, it's a set of agreements and customs that most countries follow, most of the time.

These agreements cover the rights and duties of states about things like war, human rights, trade and diplomacy.

They can be treaties: written agreements that countries sign like a contract.

Or they can be what law calls 'customs': these are principles and ideas that many already follow.

These agreements only apply to countries, not companies or individuals.

And this can get complicated because some corporations are bigger than countries.

All this means international law has to keep evolving to fit our changing world.