Celery is one of popular Mediterranean herbs recognized for its strong aromatic flavor that it imparts to variety of cuisines. (...) Wash its leaves and stem in cold water in order to remove any surface dirt, fungicide, and pesticides. Since the herb is high in fiber contents, remove its tough stem ends and chop stem and leaves closely to cook thoroughly. Its leaf tops, root, as well as stems are being used in cooking in many European, and Asian cultures. To prepare the root celeriac, trim off its top and base ends. Peel the outer tough skin using a paring knife. Cut into cubes or slice it and rinse soon in lemon or orange juice to prevent discoloration (oxidization).Celery is a Green Vegetable, it consists of the stems and sometimes the leaves of the celery Plant. Celeriac is a closely related root vegetable where the root is eaten. Celery is rich in dietary fibre and many other nutrients, it's believed celery can protect against a variety of diseases including Alzheimer and Cancer. 
Celery and celeriac can be eaten raw or cooked.  Celery can be frozen but freezing it changes the flavour.