USB-to-Serial converter based on the Prolific PL2303HX chip. For debugging, firmware updating and general serial communications.
USB A plug to 4 flying leads fitted with female header connectors - Red->+5V supply, Black->Gnd, Green->TX, White->RX.
Uses 3.3V logic.
Windows and Mac drivers can be downloaded from http://www.prolific.com.tw/US/ShowProduct.aspx?p_id=225&pcid=41
If you have any difficulties with the Windows 10 driver see here
No driver needed for Linux systems.