Parliament consists of an elected lower chamber, the House of Commons, and an unelected upper chamber, the House of Lords. In the past, the right to sit in the House of Lords was restricted to those who held hereditary titles, known as peerages, and to senior bishops of the Church of England. Today, most members of the House of Lords are life peers, appointed by the monarch on the advice of the prime minister and an independent commission, whose titles are not inherited.
Draft legislation, known as a bill, may be introduced in either chamber, but revenue bills must originate in the House of Commons. Bills proceed through a multi-step legislative process, including a committee stage and a reporting stage. Both chambers must approve a bill in the same form before it can receive the Royal Assent and become a legally binding Act of Parliament.
The House of Lords serves primarily as a venue for scrutinizing and refining proposed legislation. The Lords no longer have the power to block revenue bills, and their ability to reject other types of bills supported by a majority of the House of Commons is limited. For a more detailed description of the legislative process, consult the websites of the UK Parliament and the Cabinet Office.
Source: UK Parliament