The Romanticism is first of all a stylistic revolution. The poetic diction that characterized the classical elegy (according to which the language of the poetry must be different from the common language with different and particular vocabulary) is completely rejected by the romantics. The romantics rejected every rules and they focused on the individuality and on the free language that should be suitable for the personal vision of the poet. While the Classicism focused on the urbanity, on the city, on the society, the Romanticism focused on the individuality, on the countryside, on the nature and while Classicism means rationality, the Romanticism means imagination. So the Romanticism is a revolution because we can see a total rejection of the poetic rules. The most used verse in the Classicism was the "heroic couplet" (rima baciata), now the new romantic poets started to use a free and personal poetic language.
The word "Romanticism" comes from the word "romance". This word refers to the medieval age when the "romance languages" (lingue romanze) were born, after the fall of the roman empire. This type of languages will become the national languages; for this reason the romantics wanted to come back to the medieval ages to recuperate the origins of the national folks (unlike the Enlightenment which rejected the medieval age and studied the classical wolrd).
In England there is an exact date that mark the beginning of the Romanticism: the 1798. This was the year of publication of the "Lyrical ballads" written by William Wordsworth and Samuel Taylor Coleridge. Already in the title we can drow two important element that will characterized the Romantic movement: the focus on the subject (marked by the word "lyrical" that refers to an interior, intimate, subjective and personal world), the interest for the medieval age and so the need to take back again the medieval poetic form of the ballad (marked by the word "ballads").
The two poet that work together to write this poem were very different from each other; in fact they had two different finality. The finality of Wordsworth was to make uncommon an common thing; so his start point is the natural and he used the process of "recollection in tranquility" to make an ordinary thing extraordinary, while the finality of Coleridge was to make an uncommon thing common; so his start point is the supernatural and he used the process of "suspension of disbelief" to make an extraordinary thing acceptable to the human mind.
So we can say that Romanticism is the recombination of a world made of reality and imaginary.