There's an enormous diversity in Indian food - far more than most westerners used to their local Indian restaurants will be used to. Each state has their own distinctive styles and dishes. India is a delight for vegetarians, there's simply nowhere better to eat if you are a vegetarian! Meat eaters may find the variety a little less impressive - beef is not served most places in India, and pork is rare. Again though, this varies by state and region.
Most Indian food is quite spicy, it's almost a requirement in India that everything has a bit of spice to it - even tea and toothpaste! This can make things difficult sometime - restaurants that deal with lots of foreigners may automatically cut back on the spice, even if you want it and ask for it!
Curries
Of course Indian food is know for its curries. Curry is just a style of food preparation. Not all curries are spicy, there are hot ones, cool ones, sweet ones, sour ones... etc. Typically its eat with rice or bread.
Breads
India has wide range of types of bread - most of which are variations on flatbread. Naan is the most famous perhaps, but there are many other types and many foods pair best with a particular kind of bread. Be sure to try as many as you can, you won't regret it!
Dal
Dal is perhaps one of the most ubiquitous of Indian foods - a sort of soup made from lentils, eaten with bread. It is delicious and generally quite cheap. Because dal is prepared with plenty of boiling, it's generally safe for western stomachs. Be careful, sometimes it can include hefy amounts of oil, ghee or cream.
Drinks If alcohol is your thing, India may be a bit of dissapointment. In some states, it's not available at all - and in others locally produced spirits and beer are quite affordable. But imported alcohol tends to be pricey. Many areas have their own local specialties that are worth trying if you're adventurous - for example, in Goa there's Fenni - made from cashews. It's slightly sweet with almost an apple flavor, and can pack quite a punch! Make sure to buy it in reputable places to be sure of the quality.