Regulation - How the Agency turns its mandate, which is often broadly stated and sometimes nebulous, into concrete actions and policies. An example - I worked at the DOJ as an intern. There was a law passed to provide benefits to the families of public safety officers who died in the line of duty. What those benefits are? Who qualifies as a public safety officer? If a fire fighter gets in a car crash on his was home from a shift, does that qualify? The answers to these questions are determined by lawyers and experts in equal measure - the experts to determine what should be done to solve the problem, the lawyers to make sure the agency actually has the power to enact the policies the experts recommend. To put it in OOP terms, regulations are instances of law.
The stuff you mentioned - who is allowed to sign form 22a, minimum size of cleaning closets - is still law. The agencies also have the power to determine penalties for breaking regulations, not all of which involve going to jail. This power is again derived from what powers Congress assigns them when it writes the law.