Open fermentation is simply leaving the wort to ferment with nothing more than the Krausen to separate the wort from the surrounding air. While many breweries and homebrewers today rely on covered fermentation vessels with airlocks, some breweries such as Brewery Ommegang and Samuel Smith's have traditionally used open fermentation successfully and in the case of Samuel Smiths, have done so for centuries.
The argument as to why open fermentation does not increase contamination over a closed fermentation system is two-fold. Primarily, under proper conditions the Yeast that are pitched into and subsequently colonize the wort dominate the fermentation process preventing other ever present microflora from getting a foothold in the wort. Secondly, the thick Krausen is said to act as a natural barrier between the wort and surrounding air.
One of the main advantages to open fermentation is that yeast may be periodically harvested from the Krausen with a sanitized spoon. This yeast can then be stored in an appropriate, sanitized vessel in the refrigerator for some time. Another advantage is that an open fermenter facilitates dry hopping, as no lid need be removed and replaced.