Celery is more correctly known as Apium graveolens and is closely related to carrot, fennel, parsley and dill. The stems can grow up to 1 metre tall, and the bulb, leaves and seeds are also edible. Wild celery (smallage) grew throughout the Mediterranean region. Modern celery was thought to have been bred in the 17th and 18th Centuries. Celeriac is a bulbous member of the same species.
Celery is referred to as a member of the 'holy trinity' of Creole and Cajun cuisine along with onion and capsicum. It is also an essential part of the French mirepoix with onions and carrots.
Celery is a good source of vitamin C and potassium.
For all this and more head to Wikipedia (http://en.wikipedia.org/wiki/Carrot) and WH Foods (http://www.whfoods.com/genpage.php?tname=foodspice&dbid=14).