There's no difference, except some people are on one side of the border and others are on the other side. This is not a legal concept. We merely use laws to express a concept that existed long before the laws. Groups of people controlling territory with defined boundaries, and enforcing those boundaries, is a concept that long predates law. Indeed, it's not even unique to humans--many animals, such as wolves and dogs also demarcate territory with defined boundaries and police those boundaries.