Cromwell was a Puritan, who opposed Charles I, the King, in the Long Parliament (so called because of its eight year duration) that first met in 1640. During the Civil War he fought for Parliament. He thought that Parliamentary leaders did not do enough to try to defeat Charles I.
Cromwell became the leader of England in 1649 by leading the New Model Army. This included opponents that held different opinions about the type and degree of changes they wanted but they all agreed that Charles needed to change policies or go.
At the end of the first Civil War in 1646, Cromwell and the Army tried to negotiate peace with Charles I. In 1648 Charles I betrayed them starting the Second Civil War and they resolved to 'bring him to account'. Cromwell was one of those who tried Charles in 1649 and sentenced him to death.
Parliament asked Cromwell to crush the remaining royalist supporters of the king, who continued to rebel, against the authority of Parliament. He did this brutally, especially in Ireland and in particular at the siege of Drogheda in 1649.
When Cromwell returned to Parliament, he found the 'Rump', the last few remaining MPs, of the Long Parliament that was still sitting. He found that he could not agree with them about how to rule the Commonwealth as to do so without a king was an entirely new and untested concept. To solve this problem, the army as the most powerful group took control and declared Cromwell Lord Protector. The title was to suggest he was not a king but in reality he ruled as such.
As Protector, Cromwell could not agree with his Parliaments and he dismissed them both. Instead, he ruled the country through his major-generals, which meant that England virtually became a military dictatorship.
He allowed greater religious freedom for Protestants, but introduced a string of 'moral' laws to 'improve' people's behaviour which banned the theatre and bear-baiting, and forbade people to drink or celebrate Christmas, among other things.
Cromwell increased the navy, which defeated the Dutch and captured Jamaica from the Spanish.
When Cromwell died, he was succeeded by his son Richard, but the Protectorate collapsed and Charles II was restored as king shortly after.