Administrative Law is sometimes called regulatory law. Government agencies develop regulations, also called rules, to help them carry out their functions. Congress first passes a law, or statute, that delegates authority to an agency, the agency creates rules, and the rules are passed and become part of law.
Rules/regulations that pass have the force of law, just like laws passed by Congress.
Regulations inform business of legal rules they must follow. Regulations affect the rights and responsibilities of citizens. They also inform the public about grants and other funding opportunities.
Who uses it?
Administrative Law in the Political System by 
Administrative Law and Process in a Nutshell by 
A Guide to Federal Agency Rulemaking by 
Regulation by 
Administrative Law by 
The Administrative Regulatory Process by