The branch of law which deals with the powers and procedures of government agencies. In the United States and many other countries, this includes rulemaking (the manner in which agencies enact regulations), adjudication (the manner in which agencies resolve disputes over agencies' actions), enforcement of regulatory policy, and many issues related to due process.
Administrative law is closely related to constitutional law in that it often deals with what powers the government has and doesn't have, and in what manner government agencies may carry out the power delegated to them by the legislature.
The regulations crafted by rulemaking processes are often extremely complex, and frequently directly refer to specific scientific, technical, and financial standards (such as parts per million/billion of pollution, safety measures for certain activities, and accounting/financial reporting procedures).
Constitutional law teaches you how government is supposed to work, in theory. Administrative law, on the other hand, teaches you how it actually works, in practice.