Usually grown as a fresh, munching vegetable, celery is used in our house mostly as a flavourful herb. Leaves and stalks are harvested and chopped up for soups and stews, and tossed fresh into freezer bags for "fresh" herbs through the winter.
Celeriac is simply celery that has been bred to grow a large root and few leaves, making a lovely winter storage vegetable that adds celery-flavour to cooking.
Celery and celeriac are umbelliferae (botanical brother to carrots, dill, Queen Anne's Lace and likewise), and a biennial with seed set in the second growth season. Start seeds early indoors (approx 10 weeks before planting out), as celery is a relatively slow germinator. Prefers cool and moist growing conditions, and transplant about 2-3 weeks before last Spring frost.