Curry is not a standard blend. It is also not an invention from India - it is probably more of a British simplification of a style. A curry - it a spice blend specific to a geographical region, a family, or even an individual.
Madras curry is a relatively hot blend of spices which celebrates the Madras region of India. I includes a large amount of ground hot chile pepper, generally cayenne. Vindaloo is also a very hot curry blend which utilizes even more ground hot chile pepper to increase the capsaicin content.
The basic curry will contain 6 ground ingredients: Cinnamon, Cardamom, Coriander, Cumin, Nutmeg, and Fenugreek. From there, one adds Turmeric to obtain the ubiquitous yellow color, and chile powder to taste. In many places, a basic curry of the 6 spices is called a "Sweet" curry. Not because it has a sugar-like sweet taste, but because it has no Turmeric or Chile and is "Not Hot".
There are also Korean, Chinese, Japanese, and African curries. Each will have a bit of one ingredient more prevalent, a pinch of this, a dash of that, and all of a sudden - there is curry! Of course, a proper curry will celebrate the region - so if you are looking at Korean curry you want a bit more chile heat. A Japanese curry will be more savory. There are even curries that include Saffron! In almost all situations, you mix the curry powder in with your dish, saute, and add a broth which carries the flavor throughout the dish. Most curries are served over white rice. In some cases, you thicken the broth, in some cases you let it remain thin like a soup. There is no need to use meat; there is no rule or requirement for beef, pork, lamb, poultry, or fish to be used.
What really irritates me when I talk about curries are the large industries who have a list of ingredients of multiple man-made chemicals, thickeners, preservatives, coloring agents, and then the single word... spices. No listing of what they use; it is highly likely you are getting more chemicals than the tasty spices themselves!!!
It is actually more cost effective to buy a curry powder that is already made. Getting just the 6 basic spices together, toasting what needs to be toasted and then grinding the whole seeds into powder is a labor of love, if not a devotion to purity. Adding Turmeric and Chile powder to taste on a basic (sweet) curry is always a better option. What is much more effective is to make sure that when you buy a curry powder, it says what is included and does not use preservatives or fillers.