1. You tell Sickbeard which shows to track.
2. Sickbeard finds nzb files for you online and hands them off to sabnzbd.
3. Sabnzbd grabs the files from your usenet feed and assembles them into videos.
4. Sickbeard fixes file names for easy consumption by xbmc
5. After the files are complete, Hazel moves them onto the drive used by xbmc.
This all is implemented in python and uses web pages for the ui. It all runs perfectly on my Mac mini but works on Windows or Linux as well.
There are numerous ways for a hacker to to set up this functionality but this works very reliably for an out of the box solution.
Jetshred had a dead simple guide to OS X set up.