The ghost pepper is grown in the north-eastern part of India. Its local name is 'Bhut Jolokia'. It is so spicy that scientists measure its heat with special machines. A jalapeño - the chilli often on pizzas - is mild compared to a ghost pepper, which is about 400 times spicier.
Why are chillies spicy? It is because of a chemical called capsaicin. Birds can eat capsaicin without feeling any heat at all - which is helpful for the plant, because birds carry the seeds far away. Mammals (including humans) do feel the heat, so most leave the plant alone. Humans turned out to be the exception. We started growing chillies on purpose.
Chillies didn't always grow in India. Like potatoes and tomatoes, they originally came from the Americas. Sailors brought them to India around 500 years ago. Indian cooks loved them so much that today it is hard to imagine Indian food without them.
Chillies are used carefully. A pinch can warm up a whole pot of dal (lentils) or curry. Different parts of India have very different food, but most regions use chillies to add a kick. Some cooks add yoghurt or milk on the side to cool the mouth down after a spicy bite.