Claude Monet, Impression - soleil levant, 1872
Wassily Kandinsky, Fugue, 1914
2) Ending: hard to say when it ended, probably the mid- 1900s when Postmodernism is being attributed to starting. Though I would say that there is still art being made today that falls under a Modernism definition, like abstract sculpture and paintings that decorate corporate office buildings.
3) Anyway, it's main defining characteristics are experimentation within artistic media. in painting they played with color expression and combinations and the portrayal of light, and in sculpture as well there was abstraction and basic colors and geometry. A lot of the movements within have to do with abstraction. Modern Art went along with many posits of Modernism - thought of the times that was broadly about rejecting traditions of the past in favor of new ways of expression that weren't so restricting.
4) Main artists are many and include: Monet, Van Gogh, Matisse, Picasso, Edvard Munch, Expressionists like Kandinsky, sculptors Alexander Calder and Giacometti and Henry Moore, Abstract Expressionists de Kooning and Pollock, Minimalists Richard Serra and Donald Judd and Mark Rothko.
The only big critic of Modern Art I know of is Clement Greenburg who wrote a lot about the idea of art being an aesthetic and uplifting experience, and the art that did this was usually the most abstract, without any cultural or historical markers that would impede the transcendent art experience.
5) The aesthetic character of Modern Art includes bright and vibrant colors, "loose" painterly gestures, abstraction of figures and things, and complete abstraction of any subject, a lot of geometry-based patterns and figures and objects as well.
