What are Chekhov's contributions to the short story form?

Anton Chekov was trained as doctor, but suffered from
tuberculosis, which at the time was an incurable disease. Because he experienced success
as a short story writer and playwright, he rarely practiced
medicine.

Chekov's first group of writings was published in
1886. His first play was staged in the year following. Towards the end of the 1800s,
Chekov began working with producer/actor/director Constantin 
Stanislavski,
and this professional collaboration lasted until Chekov's death in
1904.

In terms of the short story, Chekov is seen as
greatly responsible for the form of the modern short story form that reflects the
following characteristics:

• mood and symbolism
  are more important to the story than the development of the plot (known as the
  "plotless" story).


• impressions and ideas are thematically
  arranged for effect, rather than relying on climax and
  resolution.


• Much of Chekov's attention was directed to
  those who were landowners or members of the professionally-centered middle class.
  

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Using themes relating to
the everyday life of the landed gentry and professional middle class, Chekhov
portrayed the href="http://www.merriam-webster.com/dictionary/pathos">pathos of life in
Russia before the 1905 revolution: the futile, boring, and lonely lives of people unable
to communicate with one another...

...his stories, are
studies of the spiritual failure of characters in an aristocratic society that is
disintegrating...

Furthermore:

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Chekhov combined a keen use of brevity—gleaned
from the stories of Guy de Maupassant—with a poetic and symbolic sensibility,
culminating in new casts of short fiction, specifically the plotless
story.

Chekov was also
influenced by Tolstoy, telling satirical stories from the viewpoint of
children.

Chekov put a new spin on the traditional short
story format, creating his own style.