The D1 Mini is a breadboard-compatible WiFi development board based on the ESP8266. This board is compatible with the Arduino IDE and with NodeMCU.
All of the I/O pins have interrupt, PWM, I2C, and one-wire capability, except for pin D0.

The board comes with three different sets of header pins: long-pin female headers, standard-pin female headers, and male pin headers. You can solder the set you choose to your board. The long-pin female headers are our favorite since they allow you to plug the board into a breadboard while also having female headers available on top of the board.

The board includes an on-board USB Micro connector and USB to serial (UART) converter, making it easy to power and program the board by simply connecting it to your computer with a USB Micro cable.

There is a similar board to this called the ESP8266 D1 R2 V2.1.0 WiFi Dev Board that is shaped like the common Arduino UNO footprint.

Here is the WEMO's site to get more information in all the types of
D1 MINI. Link here

Other reference sites for ESP8266 D1 Mini
- Source 01
- Source 02