A common ingredient in Thai cooking, the Thais use the leaves of the coriander plant much more than ground coriander seeds. Americans refer to the leaves as 'cilantro' and seeds as 'coriander', but in Thailand the words are the same and the plant has only one name. Throughout this site we refer to the plant as coriander, the leaves as coriander leaves and the seeds as coriander seeds. If in doubt its the leaves (cilantro)! In the above photograph, each leave is approximate 2cms across.
Coriander leaves are best bought and used fresh. However if they're difficult to come by, you can buy bulk coriander leaves and deep freeze them. Like that they can last for 6 months or longer. If you grow coriander (it really is very easy to grow), it can be harvested fresh during the summer and some set aside in a freezer so you have it during the winter months too. If you really can't obtain coriander, use parsley, however the taste is not quite the same.