I think it's impossible to classify Modern Art, at least based on what I've been seeing recently. I like some modern art, most significantly Kandinsky. Calder's kinetic sculptures are amazing also. Artists who do wierd random shit then call it art can be entertaining but I don't find it too enlightening.
For a good lesson on modern art, I'd recommend anyone to go to the San Francisco Museum of Modern Art. They've got a great range of really good art and very little of the random shit. Life-size plaster model of Michael Jackson and Bubbles notwithstanding.
EDIT: According to wikipedia:
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Modern art is a general term used for most of the artistic production from the late 19th century until approximately the 1970s.
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So the term refers to time-periods rather than anything stylistic.