A rhizome Curcum longa, belonging to the Zingiberaceae family, along with ginger, galangal and krachai. The plant has large leaves that are used to flavour some Indian and Malaysian dishes, and strikingly elongated purple/pink flowers. It is below ground however, where most of the action is.
The rhizome has a dull brown skin, with regular segmented rings along its length. Roughly the same size as an adult human finger. It bears a passing resemblance to a small knob of ginger. Once cut however, there is little doubt this is a totally different spice. The interior of the rhizome is a brilliant neon orange. Beware not to get too much on your fingers, I have scrubbed for days and still owned a pair of Oompa Loompa hands.
India is the major producer of the spice, growing around 180 000 tonnes annually, of which nearly 90 % is used domestically. It is possible to buy fresh turmeric in India and South East Asia (as well as Asian markets abroad), but it is mostly found in its powdered form. Almost all commercial curry powders contain some turmeric. To powder turmeric, it must be first cured. This involves boiling the fresh rhizome for half an hour, then drying in the sun for 2 weeks, until it is dry and brittle.
On the culinary front, apart from the aforementioned curry powders, turmeric is often used in rice dishes, especially festively vibrant yellow pilaus. Although the fresh spice is orange, once cooked it imparts a bright yellow colour. This is one reason not to substitute turmeric for saffron, cooked saffron is deep orange in colour and they have totally different tastes. The flavour of turmeric is fairly earthy and mellow, diminishing the longer the spice is cooked.
There is a related white turmeric C. zeodaria, known as zeodary, or zedoary that is used mainly in Thailand. It has a similar external appearance, but is a much paler yellow/white inside. It is also treated in a completely different manner, Thai cooks generally chop fresh zeodary and add it to nam prik (relishes) or salads.
As you would expect, possessing such a vivid colour, turmeric has been used for centuries as a dye. Most famously for Indian silk, but also for colouring paints and varnishes.