It can be classical or contemporary, narrative or abstract, tragic or comic, pretty or ugly. Some ballets are showcases for virtuosity, while others tug at the heartstrings. And shoes are no longer a clue. Ballet can be danced in any number of shoe styles or even barefoot. Now, the actual categorization is almost irrelevant.
Dance is more eclectic, more catholic, and consequently often more vital than ever.
But for the purpose of trying to understand and appreciate the form, the most important hallmark of ballet is the technique. A specific vocabulary Ballet technique, more so than modern dance, is precise. It is founded on a very specific vocabulary of basic steps and positions of the arms that you will see over and over again. The style may change from role to role, depending on what a given movement sequence is supposed to convey, but the building blocks are clearly recognizable, even in contemporary works in which the vocabulary is fused with other artistic principles. It is a traditional language that is internationally understood, with which a choreographer constructs a dance - ordering, adapting, even changing movements to suit the eye, the heart, and the individual performer's ability.