The Life Peerages Act 1958 altered the composition of the House of Lords because it introduced more people from different professions, and more women. Before the Act, the House of Lords had been made up exclusively of hereditary Peers. A life Peer differs from a hereditary Peer in that the receiver cannot pass the title on to his or her children. Although life Peers are appointed by the Queen, it is the Prime Minister who nominates them. By convention the Leader of the Opposition and other party leaders can propose a certain number.
Further information on this subject can be found from the following links.
Alphabetical List of Members of the House of Lords
Lords by party and type of peerage
Related glossary term: Peer (Member of the House of Lords)