Created by the Parliamentarians to oppose the King's forces. It was a professional army, in contrast to the part-time local militia they had at their disposal previously. The men were full-time soldiers and liable to be sent to fight anywhere. Generally, the leaders were barred from holding a seat either in the Lords or the Commons which made the army more independent of these authorities. There were 4 exceptions to this "self-denying ordinance", including Oliver Cromwell.
Commander-in-chief was Sir Thomas Fairfax. Men and their leaders tended to hold radical political and religious views. It was a meritocracy - a man would be promoted due to his abilities rather than his social position. The Army was disbanded on the Restoration.