"In English, 'to be' can have different functions:
It talks about Identity: The cat is my only pet, The cat is Garfield It talks about belonging to a class, or a group: The cat is an animal It can talk about properties: The cat is furry It can be an auxiliary verb: The cat is sleeping, The cat is bitten by the dog It can talk about existence: There is a cat It can talk about location: The cat is here" -- "http://simple.wikipedia.org/wiki/E_Prime