WeMos D1 R1 is an Arduino like development board based on the polpular ESP8266 IOT chip. It is the same Wi-Fi board like other ESP8266 boards with the convenience of Arduino form factor and an on-board usb-serial converter (CH340) . That means you can attach Arduino shields on top of it. Moreover, it can be programmed using the Arduino IDE as well.
If you are developing an Arduino based IOT project, this board is the best choice for you.