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, an avr, an msp430 or an stm32 for example.
It's very cheap compared to the ft232 alternative and works very well, up to more than 1mbit/s.
This is the pin compatible upgrded version of the ESP-01 chip.
ESP8266 is a highly integrated chip designed for the needs of a new connected world. It offers a complete and self-contained Wi-Fi networking solution, allowing it to either host the application or to offload all Wi-Fi networking functions from another application processor.