Administrative law is a huge area of law. This is true because of the breadth of the definition of this type of law. Administrative law can be defined as the law that touches on A) how administrative agencies are set up and how they are given their power and B) on the regulations that they write as part of their mandate. In other words, it is the law that is related to the creation and operation of administrative agencies.
Administrative agencies are more commonly known as government bureaucracies. These are the agencies of the executive branch that carry out the laws passed by Congress. These agencies tend to have a great deal of power because Congress empowers them to write rules and regulations that will put into effect the laws that Congress passes. This means that administrative agencies are almost as important in creating the meaning of a law as Congress is.
Administrative law, then, deals with how these agencies are set up. It also deals with all of the rules that they write. So, for example, if the EPA writes a rule about the regulation of the coal industry, any issues that come up with regard to that rule are part of administrative law.