MI stands for Military Intelligence and the differences between the two organisations MI5 and MI6 is vast. They are answerable to different departments and completely different people. MI5 is answerable to the Home Secretary, and is an indirect confirmation of the organisation's focus on domestic as well as external threats to the country.
While in contrast, MI6 answers to the Foreign Secretary in part due to its exclusive focus on external threats.
The two organisations have their headquarters in different locations. MI5 is based at Thames House. While the offices of the MI6 are located at Vauxhall Cross in London on the south bank of the river Thames. Sometimes referred to as SIS the ‘Secret Intelligence Service’.
The budget and operations of the MI6 are understandably cloaked in secrecy. While in comparison, MI5 is relatively more transparent. To fulfil its mandate, MI5 concentrates its focus on several key areas, terrorism, both domestic and foreign that takes top spot on its list of priorities. This is partly due to the rise of global terrorism as a serious challenge to the security of the United Kingdom and its interests globally. To effectively respond to these threats, the organisation spends a large percentage of its budget on non-domestic counter terrorism. The rest of its budget is shared out almost equally between counter espionage and domestic terrorism.
MI5 also works to counter subversion and the proliferation of weapons of mass destruction.
To explain it in layman terms, MI5 is for domestic intelligence, and MI6 is for international intelligence. Put more crudely, MI6 are ‘Our’ Spies while MI5 is there to catch ‘Their’ Spies. Then it gets a little more complicated in that MI6 has its own ‘Counter Intelligence’ section.
'MI5 / MI6' were the original designations given when both organisations came under the War Office, now the Ministry of Defence. Although they’re official names were acquired way back during the 1930s.