Administrative law is a very pertinent area of study for anyone who follows closely the actions of government and how it affects the people as a whole. The best place to start in defining administrative law is to consider what its goal is, by its mere existence.
Administrative law serves the purpose of defining legal powers of government agencies.
And the laws that define the legal powers of government are summed up as administrative law. The law guides the agencies to enforce their powers and use appropriate action to punish those who disobey administrative law. A statute, or law, has created the agency through the legislative branch of government. This law, which created the agency, also enacts rules which govern and these are called substantial rules. Those rules which the agency itself would create would be called interpretive rules.