What do vegetarian restaurants generally serve?

A simple answer like meatless foods helps some but it is necessary to realize that vegetarians come in varying degrees of "meatlessness". You should ask the vegetarians you are preparing the food for what their own personal limits are.

Some vegetarians just don't eat meat (cattle, sheep, chickens etc.) but will eat animal products (eggs, cheese, butter). Some exclude meat but will eat fish. Some exclude food with any relation to animals including some types of fish oil based vitamins. Jains (an Indian religion) are vegetarians but also exclude root vegetables and seeds that represent entire living plants or potential plants.