The Opposition are the political parties other than the largest or government party. They are called the Opposition because they sit on the benches opposite the Government in the House of Commons and House of Lords Chambers. The largest of the Opposition parties is known as Her Majesty's Official Opposition. The role of the Official Opposition is to question and scrutinise the work of government. In the House of Commons the Leader of the Opposition is the title given to the leader of the second largest party. He or she is entitled to a salary and sits close to the despatch box on the Opposition Front Bench. In the House of Lords the Leader of the Opposition heads the main party opposing the Government.
Further information on this subject can be found from the following links.
Her Majesty's Official Opposition
Political Parties Represented at Parliament