Apple has been highly criticized for the glass panels it uses on the front and back of its smartphone. The design was introduced in 2010 with the iPhone 4, and has obviously stuck around with the unchanged iPhone 4S.
The glass casing means that the handset isn't very durable. Luckily, cases can help protect it from drops and other accidents. In fact, Case-Mate makes one that will protect the iPhone from being run over by a large SUV...