To arrive at beer, you need a minimum of four ingredients: malt, water, hops, and yeast. The first three contribute various elements to the finished product, but it's the last one that actually makes things swing. Yeast is a living, single-celled creature (a fungus, actually) that rather miraculously gobbles up the sugar in the malt and excretes alcohol and carbon dioxide.
We tend to focus quite a bit on the kinds of malt and hops that go into a beer (sometimes even the water), but the more you learn about beer, the more you realize what a profound effect yeast has on the final flavor. It is also the one secret ingredient--as a living culture, yeast strains change and evolve and become part of the signature flavor of a brewery. I've had brewers give me a scandalized look for even asking what kind of yeast they use. Yet it is the yeast, as much as anything, that gives a brewery's beer its character.
The fascinating thing is that, as a homebrewer, you can sometimes just appropriate that ingredient whole cloth. Many breweries bottle-condition their beer, relying on the living yeasts in the beer to produce just enough waste for carbonation. After the fermentation process has ended--yeast has turned all the convertable sugars it can into alcohol--breweries dose the beer with just a dash of sugar or malt before they throw it in the bottle, to give the yeast a little more to work with.
It's actually possible to take that small amount of yeast and culture it up to quantities large enough to pitch in homebrews. After wondering for years how well this would work, I've taken the plunge, and have started to work up the yeast from that bottle of Saison Dupont I reviewed a couple days ago. So far, so good. I started out adding about 2 ounces, and yesterday bumped it up to about ten times that amount. One more stage and it should be adequate to pitch in a standard five-gallon batch.
I also picked up a bottle of bottle of Panil, which is actually an Italian version of a Flanders Red (purpoted to be every bit as good as Rodenbach, which I couldn't track down), and Cantillon Organic Lambic for additional culturing. Reviews of those as I need new yeast.