The Under $20 A Bottle Wine Thread:

[TheRiddick]

Look here: http://en.wikipedia.org/wiki/Brettanomyces

In general terms, its a spoilage bacteria (although actually a yeast). Like I said somewhere above, if you've ever been to a horse stable you know what it smells like. And once it blooms in a bottle, it takes over all other flavors leaving only the flavor of brett to "enjoy".