Curry powder is a mixture of spices of widely varying composition developed by the British during their colonial rule of India.
The word "Khari" from which "curry" is derived, comes from Southern India and refers to a sauce of any kind. "Curry powder" was developed by the British, who wished to take the taste of Indian food home, without having to utilize fresh spices. As a result "curry powder" in the Western world has a fairly standardized taste, but there are literally millions of curry flavors in India. 
The spice mixtures which came back to England during the imperial reign were very few and far between, and thus did not have a major impact, if any, on today's spice culture in Britain.
Curry powder was largely popularised after the last world war, when immigrants from South East Asia flooded to the UK to help rebuild the economy, however, even at this stage, the Curry Powder was not 'standard' as each household had its own special blend.
The late 60s and early 70s saw a rapid increase of Indian food consumption by the UK populace, this also led to the rapid increase of 'Indian' restaurants, as such, the tradition of keeping an own special blend of curry powder simply became uneconomical, and the standard Curry Powder was born.
Indian cooks have ready access to a broad range of fresh spices, from which they are able to make their own mixtures. Indeed, most Indian cooks will have their own specific mixtures for different recipes. These are often passed down from mothers to daughters.
Most recipes and producers of curry powder usually include coriander, turmeric, cumin, and fenugreek in their blends. Depending on the recipe, additional ingredients such as ginger, garlic, fennel seed, cinnamon, clove, mustard seed, green cardamom, black cardamom, mustard seed, mace, nutmeg, red pepper, long pepper and black pepper may also be added.
It is also the name given to a different mixture of spices that include curry leaves, which is used in South and East Indian cuisine.
- ↑ Hidden India: The Kerala Spicelands; pbs.org