From the Tamil word 'kari', meaning 'spiced sauce', comes this catch-all term, used to refer to any number of hot, spicy, sauce-based dishes. The term curry itself isn't really used in India, except as a term adopted by the British to categorise a number of different Indian soup or stew dishes, nearly always containing onions, garlic, ginger, turmeric, chilli and vegetable oil.
Curries from South-east Asia require different seasoning from Indian curries. Thai curry pastes tend to be hotter and more fragrant, and are flavoured with fresh chillies. You can read more about Indian cooking here.