As well as
Acts of Parliament, UK law is also made through Statutory Instruments (SIs) (also referred to as delegated, or secondary legislation). These are laws which are written by a Government minister, exercising legislative powers delegated to him or her by Act of Parliament. Some of these must be approved by Parliament before they can become law, others need only be laid before Parliament a certain number of days (usually 40) before coming into force. Some are subject to no Parliamentary procedure at all.
They are used because they are much faster and simpler to implement than a full Act of Parliament. SIs are sometimes described as "secondary legisaltion, not second class legislation". They have the same force as an Act of Parliament, and much of the UK's law is made in this way. There are literally thousands of SIs each year, compared with a few dozen Acts.