Howdy!
The advice you've gotten here is good, I'm hoping I can add a bit more. Cask conditioning is letting a second fermentation happen in the cask, just like with the bottle. However, you usually vent most of that pressure off serve it with a bit over a volume of pressure. For details, check out the
CARMA website. The purpose is to use a non-CO2 dispense method, both for tradition and to accentuate beers that go well with a 'controlled rot.' This has traditionally been British beers, but at the UK festivals this summer, CAMRA was very proud of some cask conditioned lagers around.
As Mylo said, your beer will spoil as it ages with the air, but usually it takes about 5 days to spoil if you keep it at cellar temps (around 50F) and you get a neat range of flavors over those 5 days. You have a firkin (or a pin if it is closer to 5 gallons). In the picture you have the hard spile, which you leave in during the secondary fermentation. If I recall correctly, you then replace that with a soft spile about 2 days before serving, which is when the cask 'vents'. It is imperative to let the beer sit 'in stillage' for a day or 2 before serving to let the yeast fall out and the beer run clear-- think bottle conditioned home brew on a massive scale, a shaken bottle is a bit milkshakey. You'll probably gravity feed through a spigot, which is great fun, and draws air through the soft spile to replace the gravity fed beer.
I'm jealous of your firkin/pin there. I'm thinking about doing some small-scale cask conditioning in 2 Gallon Mr. Beer casks ($10 from their website).
These guys, have a fantastic how-to. And this
company has all the fixin's for your cask beer experience.