What is the difference between "curd" and "yogurt"?

In India, curd is marketed as "yoghurt."

see http://en.wikipedia.org/wiki/Curd

Curd and Yoghurt are indeed used interchangeably in many parts of (mostly urban) India and the region in general, though probably not in the UK, the US and some other parts of the world. (Thanks to @Peter Shor for the guidelines).

This answer seems to do a good job of explaining the difference. Yogurt and curd are similar in that bacteria are used to produce lactic acid to thicken the milk. They're different in that yogurt is made with pure culture for consistency.

They are totally different things. I am a Turk and both products are highly consumed in Türkiye. We call curd lor in Turkish, and it is very similar to cheese:

On the other hand, yoghurt is a different product.

Both are made of milk.

In American English,

yoghurt (or yogurt)

is a milk-based food that is cultured with yoghurt-specific bacteria, is a soft and smooth, ranging from jello-like to almost pourable consistency served in a tub, usually eaten at breakfast time. It is usually mixed or served with fruit. There are hundred's of commercially available varieties.

curd

all by itself is rare in AmE except in technical circumstances. For the food 'cottage cheese', where milk has formed into 'curds', small lumps of congealed milk solids, by chemical processes (not bacterial ones). Cottage cheese is often labeled 'large curd' or 'small curd' (depending on the size of the gobbets of coagulated milk).

One would never mistake 'yoghurt' for 'curd' because yoghurt creamy smooth and sweet/sour, and 'curd' is a technical descriptor for a congealed bit of milk solids an sounds weird as singular. 'Curds' are what you might describe cottage cheese, and are lumpy bits, tasting cheesy.