Haydn is traditionally considered the father of the symphony? and [string quartet]?, and he did write the first well-known works in those genres. His early work is almost Baroque in style, then develops into early classicism. His works of the late 1760s and early 1770s are particularly interesting: they're in a style called "Sturm und Drang" (storm and stress), full of jagged chords, abrupt transitions, and weird minor-key harmonies. Most of the symphonies numbered between 35 and about 55 are of this kind. Abruptly -- some guess his employer objected -- he abandoned this style and became very placid and conventional for a while, gradually developing the civilized, urbane wit for which he would become famous in his last years.
In the 1790s, having become famous for his works distributed throughout Europe, he was invited by [J.P. Salomon]?, an impresario, to visit London. He went twice for periods of over a year each, writing the 12 "London Symphonies", including the "Surprise Symphony" (No. 94), which contains a sudden loud chord in the quiet slow movement.
His best-known vocal work is a very late oratorio?, The Creation.
Haydn was a friend of Mozart, and the two admired each other's work greatly. (Strangely, Haydn looked just like [F. Murray Abraham]? as the jealous, untalented Antonio Salieri in the film Amadeus?, while Salieri in reality didn't look like that at all.) He taught Beethoven briefly, but they didn't get along.