ESP8266 is a very popular WiFi enabled microcontroller. Thanks to very attractive price and huge possibilities quickly became a very popular choice - especially in domain of home automation. However bare ESP8266 is a bit tricky for beginners. It requires stable power supply, and sophisticated programming method. Wemos D1 mini solves all those inconveniences. Basically it's very similar to NodeMCU module. The main difference is size. Wemos D1 is smaller, and have newer version of wifi module - 12F. More stable and with a better range.
D3 to G and reset button to enter bootloader mode