I suppose it depends where you are, but here, I don’t think I’ve met anyone whose had a Japanese curry, yet Thai curries are all the rage.
Re: Curry leaves vs curry plant. Just want to chime in to agree with Ichbin. Curry leaves are not used in all Indian cooking, but they are a common herb. They are useless in dried form. Interestingly enough, there’s a compound in curry leaves which seem to help control diabetes. Scientsts are looking in the matter.
Curry plants, meanwhile, are a European herb, so-called because of their strong curry scent. On Margaret Island in Budapest, there was a section of the part covered with these plants. It smelled like you were walking past a curry house. Anyhow, these are not used in Indian cooking, but are edible and can be chopped up and mixed with mayo for a really nice potato salad dressing.
A typical Indian curry masala (spice mix) starts with toasted coriander seeds and to this, spices such as cumin, ginger, fenugreek, cardamom, cassia, cloves, pepper, aniseed, turmeric, mustard seeds, etc are added depending on what variety of curry you’re making. Coriander seed tends to be the predominant spice, with cumin running close behind, but this is not always the case. Turmeric lends the familiar yellow color (but is not necessary and, IMHO, way overdone) of most American and continental European curry powders. It also supposedly aids the digestion.
These curry spices are normally mixed with oil and added to a base of onions, possibly ginger, and sometimes tomatoes. Some curries will also have yoghurt, cream (north India), or coconut milk (South India) added.
Thai curries, meanwhile, are usually heavily coconut-based. They often start with a paste made of lemongrass, lime leaves, chile peppers, galangal, garlic, and dried shrimp. Occassionally you will also find tamarind paste (in sour curries and masaman curry, as well as in northern Malaysian curries) included. This paste is cooked in oil, then meat and coconut milk are added.