It's fresh, unfiltered and naturally carbonated. Some beer geeks call this smooth brew "a living beer." Some brewers call it an art form.
For the uninitiated, cask conditioned ale is a beer that starts brewing in the fermenter; then it's carefully transferred to a cask for 1-2 weeks, where it ferments a second time. You can't test the beer until the fermentation is complete. When it works, the result is brew that offers subtle carbonation and full, complex flavours. As a beer geek myself, all I can say is - try it. You'll get it.