The curry tree Murraya koenigii is a small, tropical evergreen that is native to the Indian subcontinent and Sri Lanka. Under favourable conditions it can grow to a height of 4 metres, but most specimens are in the 2 metre range.
The tree has a slender trunk that bears numerous thin stems, which droop under the weight of the uniformly distributed leaves. Curry leaves have a deep, bright green colour and grow to around 4 cm in length. When you brush against the tree it exudes a delightful citrus aroma, owing to the fact that it is in the Rutaceae, or citrus family.
The leaves themselves have no curry aroma or flavour, but take their name from the extensive use of curry leaves in South Indian and Sri Lankan cuisine. There is a totally unrelated shrub, known as the curry plant Helichrysum italicum that is native to the Mediterranean. This spindly silvery-grey plant has no culinary use, but possesses a deep curry aroma.
Although the curry tree thrives in tropical conditions, it is quite possible to grow them in temperate climates as well, so ask your local nursery if they can find you a specimen. Curry trees are often used as rootstock for citrus trees such as lemons, so they may be easier to find than you think. The fresh leaf is increasingly becoming available in specialist greengrocers, but the dried leaf is the most common type found in the west. Look for pale green leaves and pass over any that are brown or black.
Curry leaves, fresh or dried are normally fried in oil to release their wonderful flavour and aroma. This is done either at the start of cooking, or towards the end when various spices are fried and added to the nearly complete dish. They provide a subtle spicy-citrus flavour to curries and leguminous dishes such as dhal.
The leaves themselves are not eaten, giving rise to an old Indian proverb. Curry leaves are likened to shameless types who keep friends for only one purpose; just like the leaves, which add an unmistakable aroma and flavour - they are eventually discarded.
It may help to give some indigenous names if you are searching for curry leaves in an ethnic market.
Hindi - meetha neem or karipattar
Burmese - pyi-naw-thien
Malay - duan kari
Thai - bai karee
A photo can be found at http://www.csdl.tamu.edu/FLORA/mi14/mi14069.jpg