Typically, food preferences are associated with religion. For example, Muslims do not eat pork (pig) as it is forbidden by the religion and also eat only halal foods. Many Christians do not eat meat during the season of Lent. Similar restrictions apply for Jews in terms of kosher foods.
For people who follow Hinduism, the predominant religion in India, the generally accepted rule is that they do not eat beef (cow meat), as cows are considered sacred. This is also reflected in airline food preferences, where "Hindu" food means "no beef" (can include other meats). However, this is not exactly true either.
As a practicing Hindu as well as having been born and raised in India, hopefully I can provide a better answer to this vexing question to many Westerners, particularly those involved in the IT profession, as they end up brushing shoulders with Indians mostly and also have to put up with their meal preferences during lunch and dinner!
Hindu Vegetarianism
Unlike many religions, to my knowledge, in Hinduism, codes of conduct are not strictly enforced. The religion, as I perceive it, is more democratic, and lets the follower decide how to follow the religion. The codes tend to be more of 'best practices' than commandments. As a result, enforcement of the religious codes are often done by means of social and economic pressure rather than the religion itself. Due to this, the dietary composition of a Hindu is more driven by where he lives rather than what the religion says. Thus, you can find that Hindus in coastal regions such as West Bengal and on the east and west coasts of Southern India consider seafood as a staple food while also being devout Hindus. In the more interior regions, chicken and at times lamb is also eaten as a staple food by Hindus.
Theory of Emotional Detachment
When asked what type of food he eats, one of my colleagues used to say "anything that does not have eyes". This is a close enough approximation for an Indian Vegetarian. I have a slightly more elaborate theory about vegetarianism, which I feel provides a better model for vegetarianism in general and Indian vegetarianism in particular. The theory essentially is that a person's level of vegetarianism depends on the level of emotional attachment one has with what he eats. In simpler terms, the more something looks or feels like you or emotionally attached to you, the less you are likely to eat it.
So, let's put this theory to test. Based on the theory, I propose the following scale of vegetarianism.
It's simple at the start - we don't eat humans (cannibalism) because they look like us the most and also are of the same genre. Up next, we have the exotic animals (koalas, panda bears, insects), weird body parts (brain, reproductive organs, eyes, nose), and animals that we are fond of or feel relative empathy towards (dogs, cats, chimpanzees).
Things start getting a little interesting from there on. The next level is consuming most kinds of red meat, typically pigs, cows, lamb, and versions thereof. This is probably the most prevalent diet throughout the world. As we saw before there are certain restrictions here based on religion, but in general red meat is the common theme. Such animals are considered eatable, as they have been used for generations for consumption, and hence we are emotionally detached with them. The restrictions at this level reflects the level of emotional attachment as well. Pigs are considered bad as their 'filthy' habitat is considered as reflective of those who would eat them as well. Similar theory is also reflected in parts within Hinduism, where scriptures say "you are what you eat" and hence you attain the qualities of the animals that you end up eating. In addition, cows are considered taboo, as in ancient India, they were used for multitude of purposes (farming, dairy, and general labor) and so the emotional attachment was high.
In the next stage are the "white meat" eaters, who do not eat red meat, but are fine with white meat - which includes many birds - chicken, ostrich, and quail to name a few. The emotional detachment with birds tend to be a lot higher as well, since they are not typically tamed or befriended (raising chickens for eggs does not count!). They also look a lot less like us and the lack of red meat is also visually less emotional to the eaters.
The next stage of meat eating is with seafood. Once again, the emotional detachment is extremely low, as sea creatures look a lot less like us. Moreover, for generations, sea creatures have been considered as staple food along coastal regions and hence have not been considered as 'true meat'. In India, however, nowadays the aversion is more due to the sight and smell than by religious preference.
After this point, we get into the more vegetable-based space. The two stages here - "Vegetarian Lacto-Ovo" and "Vegetarian Lacto", covers what I would call as the "Indian Vegetarian", a space where I fall under. In these stages, all forms of meat (aka "those that have eyes") are not eaten, while dairy products (cheese, milk, yogurt, etc.) are considered acceptable and in some cases, even eggs (technically they don't have eyes!). There are two interesting twists here - the "cake lover" - which most of my more "strict vegetarian" friends are. They do not prefer to eat eggs - scrambled, poached, or even egg strands in a Chinese Fried Rice. However, they are fine with eating cake, even though the cakes contain eggs. I believe the emotional detachment again comes into play - as long as they do not 'see' the egg in its close-to-raw form, they are fine, such as in the case of a cake. This could also be attributed to the social aspect. Most Indians are more Westernized and birthdays are celebrated with cakes. So, we are raised with a tolerance towards cakes and hence, it's not considered as 'meat'. Same goes even for fish oil that most moms give to kids when they are young for the essential Omega-3 fatty acids that are found lacking in a typical vegetarian meal.
Another twist is the "Vegetarian Jain". Jainism is a distinct religion primarily practiced in the western parts of India (most of Gujarat and areas around Mumbai) that has its origins around the time of Buddhism. Jains believe in non-violence and hence do not eat anything that is considered as harmful to other creatures. As a result, they avoid root vegetables (can potentially kill animals under the ground while plucking the vegetables), honey, and similar products. Some people who are more strict also do not eat stale food and hence avoid any fermented product more than a day old. Interestingly however, milk is considered acceptable. My guess is that taking the milk from a cow technically does not harm the cow and hence it is fine, as long as the calf is given its due share. This is probably why even in the more general Indian vegetarianism, milk is considered as vegetarian.
Inclusion of milk and at times eggs are probably the most importance differences between a vegan as it is defined in the Western world, and vegetarian, as it is defined in India. That's also why it irks me to no end whenever I end up getting tasteless vegan stuff in Continental Airlines when I request a "Vegetarian" meal (sometimes the "Asian Vegetarian" meal, which is essentially "Vegetarian Lacto", is not given as an option).
So, hopefully this gives you an idea of the nuances in vegetarianism, and helps better understand the plight of your fellow colleagues!
