The ESP8266 is a low-cost Wi-Fi chip/module with UART connections and a built-in TCP/IP stack. It has an Xtensa single-core 32-bit L106 MCU, usually running at 80 MHz.
There's a large community behind the ESP8266, some important / interesting pages are:
- ESP8266 forum
- Wiki on GitHub with pinouts, command listings, ...
- Tutorials: nurdspace; electrodragon
- PIC18 C library
The ESP8266 is becoming increasingly popular, especially among *duino-users.
Some information in the tutorials or example software online is outdated and incorrect.