The ESP8266 processor is an 80 MHz microcontroller with a full WiFi front-end (both as client and access point) and TCP/IP stack with DNS support as well. While this chip has been very popular, its also been very difficult to use. Most of the low cost modules are not breadboard friendly, don't have an onboard 500mA 3.3V regulator or level shifting, and aren't CE or FCC emitter certified
Adafruit HUZZAH ESP8266 breakout is what Adafruit designed to make working with this chip, fhey took a certified module with an onboard antenna, and plenty of pins, and soldered it onto their designed breakout PCBs.
Specifications
Reset button,
User button that can also put the chip into bootloading mode,
Red LED you can blink,
Level shifting on the UART and reset pin,
3.3V out, 500mA regulator (you'll want to assume the ESP8266 can draw up to 250mA so budget accordingly)
Two diode-protected power inputs (one for a USB cable, another for a battery)
1 x Analog input (1.0V max)
9 x GPIO (3.3V logic), which can also be used for I2C or SPI
2 x UART pins
2 x 3-6V power inputs, reset, enable, LDO-disable, 3.3V output
One breakout at the end has an "FTDI" pinout so you can plug in an FTDI or console cable to upload software and read/write debugging information via the UART.
Each module comes pre-loaded with NodeMCU's Lua interpreter, you can run commands, and 'save' Lua programs directly to the module's Flash using a USB-Serial converter cable. Or use the Arduino IDE. Once you download the ESP8266 core, you can treat it just like a microcontroller+WiFi board.
Each order comes with one assembled and by Adafruit tested HUZZAH ESP8266 breakout board, and a stick of 0.1" header that you can solder on and plug the breakout into a breadboard.
The SparkFun ESP8266 Thing is a breakout and development board for the ESP8266 WiFi SoC – a leading platform for Internet of Things (IoT) or WiFi-related projects.