It's a small USB to TTL serial tool, using the PL2303 chip. You can use it to connect some serial device to your PC via USB port.

TTL to USB Converter Module (PL2303) Specification

• Module Type : AdapterBoard
• Size : 4.6 x 1.5 x 1.1cm
• Operation Level : Digital 5V
• Power Supply : External 5V