The most important thing you should know about Henri Rousseau is that he had a completely boring life - with the exception that he became convinced, somewhere along the way, that he was a superbly talented artist. Born to a plumber, little of Henri's young life indicated he was destined for greatness. He attended school (though not art school), enlisted in the military (though he never left France) and got a respectable (though low-level) job, wife or two and nine children, and paid his taxes.
However! Despite the typical, mind-numbing day-to-day grind, Rousseau kept some unfathomable creative spark alive in his mind. Perhaps because of his day job, he'd obtained a permit to copy works at the Louvre by 1884. Though he never took a formal art lesson, he quickly developed a style from which he scarcely deviated until the end of his life. Dismissed by many - for many years - as "primitive" or "unformed" (or, even "uninformed"), his style was, nonetheless, a harbinger of things to come.
Rousseau's work is characterized by heavy dependence on line, stiff (and unrealistic) portraiture, wild juxtapositions and flattened perspective from which the Cubists and Surrealists drew heavily. His imagination plays a major role in his work; Rousseau never personally set foot in a jungle. He did, though, spend considerable time viewing the plants and animals at Paris' Jardin des Plantes. It seems wrong to label his work as "primitive" without acknowledging the sense of wonder behind it.
Birth: May 21, 1844, Laval, France
Death: September 2, 1910, Paris, France