The judges are generally known as the Law Lords although this is not their official title. The judges consist of senior judges from the Court of Appeal and the High Court who have been appointed a life Peer for the sole purpose of being a judge in the House of Lords.
The same judges (plus other judges from other Commonwealth countries form the Judicial Committee of the Privy Council which is the final Court of Appeals for other Commonwealth Countries such as Trinidad and was the court of last resort in other British Empire countries as Canada and Australia where some of the Privy Council judgments are still binding legal precedent.
The judges hear cases which are appealed from the Court of Appeal (both civil and criminal). Generally a case must be of great importance, due to its severity, complexity, level of money involved, public interest or other factors, before the Lords will hear it. For example, the Lords were asked to decide whether General Pinochet who was the former head of state of Chile had state immunity from prosecution on torture charges when he was detained in the United Kingdom. Another case involved the issue of a hospital's authority to separate conjoined ('siamese') twins against the wishes of the parents.
The Law Lords are the highest court in England, but they have traditionally had no power to declare laws invalid or unconstitutional, as do supreme courts elsewhere, though the under British constitutional law principles they could declare a law to be inoperable as being contrary to the laws of the land. However, in common with every other court in the European Union they have the power to refer points of law to the European Court of Justice and as a result of such a case the Lords declared the Maritime Shipping Act 1990 unenforceable in part.
Generally three or five lords will hear a case, but in more serious matters even more may be present — the Pinochet case had seven judges participating in the judgment.
The senior judge is the Lord Chancellor who is also head of the House of Lords and a minister in the government — it is powerful constitutional position. Because of concerns relating to the modern doctrine of separation of the powers, the Lord Chancellor traditionally does not sit in cases where the government is a party to the action.