Delegated Legislation: Executive Order or Rule Making?

Yaar, samajh nahi aaya! I'm re-reading my notes for LAWCET, and I've got a serious case of confusion. Can anyone pls clarify the difference between a delegated legislation under a particular Act and an Executive Order issued by the President/Govt.? They seem to be doing the same thing - setting up rules and regulations to govern a particular area. For eg. in M.C Mehta v Union of India [1986 SCC (3) 151], the SC had to look into the validity of the Central Government's decision to set up a hazardous waste management board via executive orders. But what about when the Act itself delegates legislative powers to a particular authority? Is it then still delegated legislation or does it become something else? Please help, I'm stuck here!