Again referencing Achaya, in his entry on curry:
From the Tamil word kari, a term for black pepper, derives the Indo-Anglian curry, which has come to symbolize Indian food for the westerner. The term originally denoted any spiced dish that accompanied south Indian food, and was first so referred to, using the term caril, by Correa as early as in AD 1502 and by Garcia da Orta sixty years later. Later the word curry was greatly widened in usage to include a liquid broth, a thicker stewed preparation, or even a spiced dry dish, all of which appear in turn in a south Indian meal, each with its own name. [e.g., rasam, kootu, poriyal - Marc]
In the tens of different Indian dishes that I have cooked, it is quite rare that the recipe has called for "curry powder." Instead, each recipe has a carefully chosen collection of herbs and spices, and cooks can spend a lifetime learning and mastering the subtleties of blending spices to create the right flavor or bring out the essence of the raw ingredients. For example, cumin, coriander, black pepper, tumeric and ginger in one stew; garlic, ginger, green cardamom, cinnamon and cloves in another.
Julie Sahni's Classic Indian Cooking has the following explanation of the origin of "curry powder":
The earliest British merchants, who arrived with the East India Trading Company, worked and settled along the southeastern coast of India. It is more than likely that they wanted to take back with them to England the familiar aromas, flavors, and colors of the Indian food they had become so passionately fond of. But not having mastered the different Indian cooking techniques or a sense of the spice blends, they in all likelyhood just indiscriminately sprinkled kari podi [curry powder] over stews and casseroles. This yielded preparations with the familiar golden color, hot taste, and flavor of the dishes known as "curries."
Finally, although the above definition of curry proposes that the word originates from the Tamil language in south India, my favorite south Indian cookbook, Dakshin, lists four other powders in the "Basic Recipes" section before "curry powder" appears (Sambar 1, sambar 2, rasam, and mysore rasam powders).
Since the subject has drifted from the curry leaf to curry powder, my standard curry powder recipe comes from Classic Indian Vegetarian and Grain Cooking, by Julie Sahni. Most Indian cookbooks have their own variation. If you have a dedicated spice grinder (I use a coffee grinder), be sure to start with whole spices, as the resulting powder will be much more vibrant than using powders or a pre-packaged mix. I don't use it much when cooking from my Indian cookbook library, but it is good to have around (to put in Singapore-style rice noodles, for example).