The 2G (or 2.5G, or 1st-gen/original iPhone) is the one with the aluminum back.
The 3G had a plastic back, white or black, with the type information in gray.
The 3GS was identical to the 3G, but with the type information in silver.
The 4 GSM version is the classic 4 design, three joints in the antenna band (two at the bottom, one opposite the mute/volume switches.
The 4 CDMA version has an extra joint in the antenna above the mute button (four joints total, symmetric), NO SIM slot.
The 4S has the four plastic joints in the antenna band, but also has a SIM slot.