Year introduced: 2011
Capacity: 8, 16, 32, 64 GB
Colours: black and white
Model number on the back cover: A1431, A1387
 
Details: the front and back are flat and made of glass, and there's a stainless steel band around the edges. The volume up and down buttons are marked with a "+" and "-" sign. There's a SIM tray on the right-hand side that holds a "third form factor" (3FF) micro-SIM card.
See the tech specs for iPhone 4s.