The ESP8266 is a small development board that includes a MOD-WIFI-ESP8266-DEV board in a socket, a 250V AC / 10A relay (RAS-0515), a button, a UEXT connector, and a 0.1″-pitch header connector with all ESP8266 signals.
The board must be powered by a regulated 5V supply via its 2.5mm barrel jack. Please see the Powering the Board notes below.
A strip of pin headers is included loose for you to solder to the GPIO header pads if desired.
The button has two uses. It's usually a general-purpose button, but if it's pressed during power up the ESP8266 will go into bootloader mode so you can upload new firmware to the chip. To upload code you will need any USB-to-serial 3.3V converter like the USB-SERIAL-CABLE-F (which you can add using the option checkbox at the bottom of this page). You can plug the leads of that cable into the appropriate UEXT pins of this board.
If you use the new Arduino IDE for ESP8266 released recently, with this board you can have an Arduino-driven WiFi device that is faster and cheaper than an official Arduino board by itself which doesn't even have WiFi built in!