MI5 and MI6 are both intelligence agencies of the United Kingdom, but they have different roles and responsibilities. MI5, also known as the Security Service, is the UKs domestic intelligence agency, responsible for identifying and neutralizing domestic threats or security threats emanating from within the UK. MI6, formally referred to as the Secret Intelligence Service (SIS), is the UKs foreign intelligence service, tasked mainly with the covert overseas collection and analysis of human intelligence in support of the UKs national security. MI6 is also responsible for spying on other countries and gathering intelligence pertinent to the UKs international affairs.
MI5 and MI6 were originally part of the Military Operations and numbered MO5 and MO6, lower numbers dealing with various administrative matters. They kept the same numbers when Military Intelligence was formed. MI7 dealt with censorship. MI5 investigates matters of national security in the UK, such as terrorists and counterinsurgency, while MI6 gathers intelligence pertinent to the UKs international affairs.
