Showdown is a simulator of Pokemon battles. It currently only supports Generation 5 (Pokemon Black and Pokemon White).
Showdown requires Node.js (and perhaps also Socket.IO).
Copy config/config-example.js into config/config.js, and edit as you please.
After this, start Node:
cd <location of PS>
node app.js
Visit your server at http://play.pokemonshowdown.com/?server=SERVER&serverport=PORT
Replace SERVER with your server domain or IP, and PORT with the server's port (default is 8000).
Currently, this workflow works for testing Pokemon Showdown and testing your patches before submitting pull requests, but I wouldn't recommend running a production server this way. I'll make it easier to host your own server in the future.