Basically, it allows you to pick and choose exactly what your want from their massive catalogue then you download a customized .zip that you add into your game.
Here's the link to the site: Painterlypack.net
Here's a link to the one I'm using (so you can try it yourself): 1302510463.7976.zip
And here's some screenshots of what our world looks like with my custom pack:
Subject: Painterly Custom Texture Pack 
Mon Apr 11, 2011 10:15 am
