ESP8266 is an UART to WiFi module, a really cheap and easy way to connect any small microcontroller platform (for example Arduino) wirelessly to Internet.
The ESP8266 is now one of the leading platforms for the Internet of Things. It’s super cheap, and super easy to work with.
This is a serial module with a built-in TCP/IP stack, so you can use it standalone
You can use AT commands to connect with WiFi networks and open TCP connections without need to have TCP/IP stack running in your own microcontroller: You can simply connect any microcontroller to ESP module and start pushing data up to internet.