Section 20 is a division of MI6.
It is defined as "a secretive branch of the British Secret Intelligence Service who operate several high risk, priority mission around the world [Wikipedia]". According to the Strike Back wiki, "Section 20 was originally formed as part of the 'Section Initiative' within the British military intelligence establishment. The unit was designed to be able to travel around the world covertly neutralising terrorist threats against the United Kingdom and its allies, able to mobilise immediately and have their mobile command station (known as "The Crib") up in a matter of hours. The nature of the unit and its secrecy also allowed the British Government to retain a certain level of plausible deniability should an operation abroad turn bad".
When we are first introduced to this department, it was closely watched and directed by the leadership in Vauxhall Cross but as time progresses, the unit becomes a tad more autonomous whose governing body seems to be in Whitehall though whether that means MI5 or MoD is uncertain.
For all its connection to MI6 and possibly MI5, those comprising the teams of Section 20 are all highly experienced members of the British military, primarily but not exclusively the Army. Additionally for reasons not really explained, several Yanks have been made part of the team as has at least one Aussie soldier from Down Under. The missions that Section 20 members will be given will be a combination of military action (a large part) and clandestine work (smaller but very important).
We will be around to see that the roster of membership in Section 20 will change over the years of our monitoring. A couple of these changes will come from the deaths of operatives while some will be because of other reasons such as physical and mental injuries. Two major players will disappear when forces from other countries decide to try to eliminate them!
Section 20 has a decent number of personnel working for it. Key among those (IMHO) are:
John Porter - former SAS operative, brought into Section 20 because he knows one of the key players in a terrorist organization the team is hunting.
Hugh Collinson - former British Army, the real reason for Porter's being essentially kicked out of the SAS, he would become the head of Section 20 for a time.
Michael Stonebridge - former Royal Marine Commando and later SBS operative, he was personally picked by the sitting head of Section 20 to be a field leader.
Damien Scott - former U.S. Army and Delta Force, when he began to question the existence of WMDs in Iraq, he was framed with drug possession and kicked out. He was brought into Section 20 because it was thought he could identify the terrorist leader they were after at the time.
Samuel Wyatt - former JSOC operator, very much a loner who does not like teams, he is recruited by a Section 20 director who knew how effective Wyatt can be.
'Mac' McAllister - former UK Special Forces, he was about to be court-martialed for hitting a superior officer but then was saved by Section 20 to work for them.
Gracie Novin - former lance corporal in the Australian Army, she became a member of Section 20 when she was needed to help rescue two other members. She is sarcastic and snippy, especially to authority, but more than holds her own in a scrape.
There are, as said, a good number more people in Section 20 and some of them play major roles in that organization but we are shown repeated that being in an organization going after terrorists is a very dangerous, sometimes fatally so, life of work.