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.