Indian cuisine

Indian cuisine is quite varied. It is characterised by it's use of a wide variety of spices and vegetables native to the subcontinent.

In the Western cultures of Europe and Northern-America, the term "curry" is often used as a synonym for Indian food. Curry, however, is not a specific Indian dish, but rather describes a gravy based dish that contains certain spices, like cumin, coriander, turmeric, red pepper, ginger and cinnamon.

Since vegetarianism is commonplace in India, the cuisine of the country includes many interesting vegetarian dishes. More from Wikipedia

Leonard and Sheldon frequently order Indian food (0101, 0221), while they avoid it when Raj is around because he detests it (0107, 0221, 0302). In The Jiminy Conjecture it is revealed that they usually have Indian food from Tandoori Palace, but in this episode Leonard switched to Mumbai Palace, which causes considerable protest from Sheldon (0302).

Leonard considers curry a natural laxative. Sheldon does not like to have to have Indian and Thai food on the same day, because they are both curry-based cuisines and he therefore considers them "gastronomically redundant" (0101).

Sheldon's favourite Indian dish seems to be "Kadhai Paneer", which he can only eat, when he's on his own (0221):

Sheldon: Not only is it Indian food, which Koothrappali loathes. It contains a generous helping of peanuts, which would reduce Wolowitz to a wheezing, 97-pound blister. And finally, its main ingredient is paneer, a farmer's cheese that would cause Leonard to render any room uninhabitable within minutes.

Note: He's referring to Howard's peanut allergy and Leonard's lactose intolerance.