The Great Awakenings were a series of religious revivals in evangelical protestantism that have taken place since the 1730's. Historians have posited three/four GA's in that time but mostly there are two periods considered classics. Although the first GA started in the UK, it was quickly transplanted to the US colonies and the second GA was a mainly American phenomenon that was transported to the UK. The first revival was of a strongly personal, salvationist commitment to God rather than a doctrinal one and was spread by charismatic preachers building on the teachings from the puritans and Scottish Presbyterians 100 or so years before. The second was more a period of inventive religious sects, but again often gathering around a charismatic leader.
The First Great Awakening was considered to be in the 1730's and quickly spread across England and the 13 American colonies. Preachers like brothers John and Charles Wesley and their friend George Whitefield who would found the Methodist Church had a huge influence on people in England and Wales. They held public services outside of the established church so were no longer restricted in how or what they could preach, but ultimately the CofE held state approval and non-denominational religion was, if no longer curtailed, was not promoted by the state. In the US, enlightenment rationalism was much more prevalent, and because of strict religious tolerance in many of the states, no one doctrine or sect commanded a majority and church attendance was low. Charismatic preachers like Jonathan Edwards were beginning to gain an audience preaching counterpoints to what they thought was stale, spiritually dead scientific rationalism and when Whitefield and the Wesley's travelled to the colonies to preach they did so to huge crowds. Some historians say that the 1st GA had a huge influence on the formation of the USA and it certainly had an influence on the culture there. Princeton University and Dartmouth College were founded by Evangelists. American Religion would henceforth be evangelical, charismatic and demand personal faith and spirituality.
The second GA was in the early to Mid 1800's and this took place in what became known as the Burned-Over District in upstate New York. The district was Named for its over-abundance of hellfire-and-damnation preaching and it produced dozens of new denominations including the Mormons, The Millerites (who would split after a failed End-of-the-world prophecy into the Seventh Day Adventists, and the Jehovah's Witness), The Oneida Community (A free-love, communitarian sect that went on to found a still trading successful silverware and cutlery manufacturer); and of course, Spiritualism (The Fox Sisters lived in the area at the time). As part of the Second Great Awakening other reform movements such as temperance, abolition, and women's rights came to the fore. Some historians class the Evangelical revival of the mid-late 20th Century as a third Great Awakening. This helps explain why there are thousands of sects of Christianity in the USA alone.