An Act of Parliament creates a new law or changes an existing law. An Act is a Bill approved by both the House of Commons and the House of Lords and formally agreed to by the reigning monarch (known as Royal Assent). Once implemented, an Act is law and applies to the UK as a whole or to specific areas of the country.
Putting the Act into force
An Act may come into force immediately, on a specific starting date, or in stages.
The practical implementation of an Act is the responsibility of the appropriate government department, not Parliament. For example, laws relating to transport issues would come under the administration of the Department for Transport.
Parliament or its committees may investigate how the government implements the Act and would consider any future Bills that amended or replaced it.
Changes to Acts
Future changes to the law happen through the passing of another Act or delegated legislation. An Act can also be repealed so that its provisions no longer apply. Parliamentary committees examine UK laws and recommend the removal of out of date legislation.
Finding the text of Acts
Looking for the text of an Act? The first question you need to answer is do you need the text of the Act in the form it was passed by Parliament or are you looking for a revised version of the Act incorporating any changes that have been made by subsequent legislation?
Writing of Acts
The UK Houses of Parliament changed from hand writing original Acts of Parliament to printing them in 1849. They are printed on vellum, and still are to this day.
The Acts were also the bills, until the switch to printing. The bills would pass through one House, be handwritten onto parchment after report stage, and all amendments made on third reading and during all stages in the second House were painstakingly made onto the parchment. This same document became the Act after Royal Assent. This is the case from 1497-1849, so anyone wanting to consult bills in this period should actually consult the original Act.
There do additionally exist printed bills for reference from the early 18th century but they weren't made systematically, there's no definitive set here, sometimes they were printed by private promoters (rather than by Parliament), and where odd bills do survive there's often no indication of what stage of debate they were made at. Their usefulness therefore varies.
Text of Acts as passed
Texts of Acts of Parliament as originally passed by Parliament since 1800 are available on the Legislation.gov.uk website. The Parliamentary Archives has copies of original Acts from 1497.