He was, like Caspar David Friedrich, a Romanticist painter who was way ahead of his time, in my opinion.
British!
He had a very long, controversial career.

And he shares part of his name with a famous movie character.

Hello, Will!
Self-Portrait, c. 1799

Turner was born in London, England, in 1775. A very ambitious chap, he was oftentimes considered (quite appropriately) to be a child prodigy considering that he was accepted into the Royal Academy of Art at age 14 or 15 and by 18 had his own studio.
He came from a well-off family, so he never had to work for money. This allowed him lots of leisure time to travel all over Europe to experience different places and study art, which he did during his twenties. Landscape painting was his specialty, but he painted other subjects early in his career, like the Salisbury Cathedral Seen from the Cloister (c. 1802).

Royal Academy of Arts, Burlington House, London. Established 1768.

So what did Turner do? He turned landscape painting upside down. It was no longer about capturing the picturesque -- he made it complicated by incorporating allegorical themes into his work. Also, his style was unlike anything anyone had ever seen before: very precise, dramatic, sharp brushstrokes... intense compositions and atmospheric effects... an interest in historical and contemporary events, some of which had very dark themes.
This is Snowstorm: Hannibal and His Army Crossing the Alps (1812). In this painting, Turner is associating Hannibal with Napoleon. Most of the emphasis, though, is on the vulnerability of the soldiers in the face of raw nature, which dominates the entire painting.

The Shipwreck, painted in 1805, possibly depicts a real event that occurred around the time. Turner loved the sea. He loved exaggerating the immense power and force of the waves against measly man-made ships. Again, he shows that humans are at the mercy of nature.


A couple more examples of his later work:
The Burning of the House of Lords and Commons, 16th October, 1834 (1834-35)

Rain, Steam, and Speed: The Great Western Railway (1844)

So... did anyone else notice any similarities between J.M.W. Turner and William Turner from Pirates of the Caribbean? I mean, their fathers were both named William Turner. They're both British. Both loved the sea and spent a lot of time around it...
That only leads me to ask... could J.M.W. Turner have been a pirate?! The world may never know...