From Wiki:
All parts of the plant are edible, but the fresh leaves and the dried seeds are the most commonly used in cooking. Coriander is commonly used in Middle Eastern, Mediterranean, Indian, South Asian, Latin American, Chinese, African and Southeast Asian cuisine.
The leaves are variously referred to as coriander leaves in Britain; cilantro (from the Spanish name for the plant) in the United States, and dhania in the Indian subcontinent. The leaves, and especially the stems, have a very different taste from the seeds, similar to parsley but "juicier" and with citrus-like overtones. Some people instead perceive an unpleasant "soapy" taste and/or a rank smell. This is believed to be a result of an enzyme that changes the way they taste coriander leaves, a genetic trait, but has yet to be fully researched.[3]
The fresh leaves and stems are an essential ingredient in many Vietnamese foods, Asian chutneys, Mexican salsas and guacamole, and occasionally is used in sushi rolls. Chopped coriander leaves are also used as a garnish on cooked dishes such as dal and many curries. As heat diminishes their flavor quickly, coriander leaves are often used raw or added to the dish right before serving. In some Indian and Central Asian recipes, coriander leaves are used in huge amounts and cooked till they dissolve into sauce and their flavour mellows.[1] Another factor that dictates the quality of flavor is the time when coriander is harvested. If its roots consistently stay at a temperature above 75 degrees Fahrenheit, the herb will quickly bolt, causing its leaves and stems to yield a bitter flavor and become quite chewy. At this point, made evident by the thinner and finer leaves, it is practical to harvest only the coriander seeds, since the stems and leaves are no longer usable as food.