Celeriac is a root vegetable, somewhat related to carrots and parsnips. It actually is the root of a variety of celery and has been refined over time into the big, lumpy root balls you'll find at Daisy Mae's at Findlay Market. Cut the thick skin off with a paring knife, and you'll find a white, creamy flesh that tastes a little like celery and a little like parsley. This white flesh can be shredded or chopped and eaten raw in salads, or stewed, roasted, simmered, or fried as an alternative to potatoes.
Because celeriac is a root vegetable, it generally starts appearing in stores and recipes as we approach winter. It stores well, tastes great when combined with other vegetables, is high in fiber and potassium and low in calories, and can add a unique flavor to common dishes. Trust me on this--Beauty is only skin deep.