The Lisbon Treaty is an international agreement between the then 27 Member States of the European Union that entered into force on December 1, 2009. It reforms and replaces the Treaty on European Union (TEU) and the Treaty establishing the European Community (EC Treaty). The alterations include an extension of police and judicial cooperation in criminal matters, greater involvement of national parliaments in EU lawmaking, the introduction of a European citizens’ initivative, establishment of the office of President of the European Council, the development of skills in the fields of foreign and security policy, the legally binding nature of the Charter of Fundamental Rights, and provisions for withdrawal from the EU.

Source: Wikipedia, edited