A USB to (3.3v) serial adaptor for use with the Raspberry Pi, with a pinout designed to plug straight onto the Raspberry Pi GPIO pins.
A simple adaptor which uses built-in USB-CDC drivers on most modern operating systems (Win10/Linux/Mac OS X/Android). To aid orientation, the triangle/square marking should be plugged into Pin #1 on the Raspberry Pi GPIO header.
Use with a micro-USB cable (make sure the cable has a data line and not just power!).