Others have touched on the different points, but I’ll try to respond from a specifically Canadian perspective. I take the OP as asking two different questions: what is the technical difference between Acts, Statutes and Regs? And, why have Acts and Regs?
We use the term “Act” to mean a law passed by the Parliament of Canada or the Legislatures of the province. This usage comes from the British Parliament. This type of legislation is “primary” legislation - that is, it’s enacted by the highest level of legislative body, which gets its authority directly from the Constitution, as Polycarp commented. We don’t tend to use “statute” as a formal term. It’s an informal term for a law passed by Parliament or the Legislatures, but it has the same meaning as an “Act.” When I’m writing a brief or a memo of law, I might switch back and forth between “Act”, “statute” and “law”, not because there’s much technical difference between them, but just as a matter of style - switching up the terms helps to keep the piece of writing from getting stultifying.
However, in our system, also inherited from the British, the Parliament or Legislature can delegate the legislative function to other individuals or bodies. And, since we don’t have a strict separation of powers in the American sense, that delegation can be to the executive.
When Parliament or the Legislature delegates legislative power to the executive, it’s typically to the Governor-in-Council (that is, to the federal Cabinet), or to the Lieutenant Governor-in-Council (i.e., the provincial Cabinet). Typically, the terminoology is that the cabinet is authorised by the Act to enact “regulations”, the term normally used for this type of secondary or delegated legislative authority. The delegation of legislative power must be express in the Act, which also sets out the subject-matter and limits of the delegated legislative power. If the Cabinet passes regulations which exceed the grant of power from the Act, the courts have the power to declare the regulations a nullity, to the extent they exceed the grant of delegated power.
As to the second question: why regs? There are a variety of reasons. One of the simplest reasons for delegated legislative powers is to handle matters that don’t really require the full attention of the people’s elected reps. As others have commented, passing an Act is a long and involved process. If every single change to the law requires the passage of an Act, then that will take up the time of the parliamentarians, which could be better spent on “big-picture” issues. So for example, it used to be the drafting practice that if a form was needed for a particiular Act, the form was set out as a Schedule to the Act. But that meant that if there was any need to change the form, even for something as trivial as changing an address listed on the form, you would need to have an Act passed, changing the form. Nowadays, one of the most common reg-making powers is to give the Cabinet the power to enact the forms necessary to implement an Act.
Another reason, as others have mentioned, is the level of detail needed to implement modern legislation. Our laws nowadays are much more detailed than a century ago - whether that’s a good or bad thing is a matter of debate, I suppose, but so far that tendency doesn’t show any sign of stopping. And the more detailed legislation you need, the more likely it will go into subordinate, delegated legislation.
Water testing is a good example. The Legislature might set out the general requirements for water-testing - when it is to be done, and other high-level types of decisions. But what actual tests should be used? That’s a more technical question, and it could change suddenly, if some smart scientist comes up with a more effective test. Or it might be that new studies suddenly show that a particular chemical is far more dangerous than previously believed, so you want to add it to the list of substances to be tested for. And what if new research shows that lower levels of a particular substance are more dangerrous than previously thought? You want the power to chagne the acceptable level quickly, to respond to that sort of situation.
So the regs typically have more detail than the Act, and also can normally be amended more quickly. If you had to wait for an Act to be passed, it could take a year or more, depending on the legislative cycle of the particular legislative body. (I’ve heard lawyers working for the federal government joke that the gestation period for a federal Act is longer than that of an elephant.) The regs-process, on the other hand, is usually much quicker.
Codes of Practice may or may be set out in regulations. It depends a lot on the particular government’s approach to regulations. Some governments are developing partner-based models, where government and the industry being regulated work out the guiding principles jointly. In that type of situation, the Code of Practice may be a voluntary one.