From WiseGeek-
History. The coriander plant was grown in the Hanging Gardens of Babylon, while cilantro is an ingredient in the bitter herbs or maror that are a traditional part of the Passover meal.
Description. Initially, the coriander plant has leaves like parsley, to which it is related, but when the plant bolts, the leaves it produces are very different in nature and no longer as desirable. Hence, two different strains have been developed -- plants that are slow-bolting and better for cilantro production, and plants which are encouraged to bolt to produce the coriander fruit. In any case, the plant has small white or pink flowers, and when grown for the spice coriander, attains an average height of 2 ft. (60 cm.), though it can reach 3 ft. (about 1 meter).
Gardening. The coriander plant is a hardy annual that does not transplant well because of its taproot. When growing it for cilantro, one way of dealing with its tendency to bolt it to use succession planting and protect it from extreme heat. Also, if you are growing the plant for its leaves, the sudden production of a long, tall stalk forewarns the plant's intent to flower.
Food and other uses. Cilantro is used as a garnish in Southeast Asian food, and as an ingredient of Thai green curry paste. In Mexico, it is used in salsa and guacamole. When used in cooking, cilantro should be added in the last few minutes so it doesn't lose its flavor.
and from Ochef-
"What is Cilantro?"
Q-In Far Eastern cuisine an ingredient called cilantro is mentioned. I'm from Sydney, Australia and haven't heard of it. Can you help me … perhaps there is another name for it? Or it could it be a spice?
A- Cilantro, also known as coriander, is a plant that is widely used in Asian, Middle Eastern, and Latin American cooking. The plant itself is coriandrum sativum, is indigenous to Southern Europe and the Mediterranean, and is related to parsley. In common usage, at least in the States, cilantro refers to the leaves of the plant and coriander refers to the seeds, which are often ground.
There is a pretty big taste difference between the leaves and seeds. The leaves have a very pungent smell and taste and are used in highly seasoned foods. In fact, the leaves have not found broad acceptance in Europe outside of Portugal, which developed an affinity for the taste following its conquests in Africa. (The leaves are also referred to as Chinese parsley and Arab parsley.) The seeds, however, have a spicy, lemony aroma and taste.