The word "Karhee" or "Kadhi" 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.
Curry powder was largely popularised after World War II, when immigrants from Southeast Asia moved to the UK. Still, curry powder did not become standardized, as immigrant households often had their own blends of curry powder.
The late 60s and early 70s saw a large increase of Indian food consumption by the UK populace. This also led to an increase of Indian restaurants. The tradition of keeping special blends of curry powder simply became uneconomical, and curry powder became increasingly standardized.
Indian cooks often have readier access to a variety of fresh spices than their native UK counterparts, and are more likely to make their own mixtures. Indeed, most Indian cooks will have their own specific mixtures for different recipes. These are often passed down from parent to child.
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, mace, nutmeg, red pepper, long pepper, and black pepper may also be added.