What is Adafruit Feather?
Adafruit Feather is a family of thin, lightweight development boards made for prototyping. They are designed to be used on their own, or are stackable for multiple functionalities. Feather boards can be both battery operated or have a microUSB port for a PSU to be connected.
There is a family of products surrounding the Adafruit Feather range to help you get your projects up and running.

More about the Feather HUZZAH with ESP8266 …
This “all-in-one” WiFi development board features the ESP8266 WiFi microcontroller (MCU) clocked at 80 MHz and at 3.3 V logic. It’s connectivity makes the Feather HUZZAH board of IoT (Internet of Things) applications. There is also an Arduino IDE to help you program your board.
The Silicon Labs CP2104 USB-to-Seral chip which has the ability to upload code at 921600Bd, perfect for high-speed prototyping and developing.

It’s portable!
You have the option to use either a 3.7V Lithium polymer battery, or connect to a PSU via microUSB. If you choose to power your Feather HUZZAH board by battery, it makes it portable. So you can take it with you, wherever you need to. If you need to recharge it at any point, just plug it in via the microUSB and it will recharge.

Here’s the facts & figures:
MCU: ESP8266 @ 80MHz with 3.3V logic/power
Memory: 4MB of FLASH (32 MBit)
WiFi: Built in 802.11 b/g/n
Power: 3.3V regulator with 500mA peak current output
USB-Serial Converter: CP2104 with 921600 max baudrate for speedy uploading
GPIO: 9 x pins - can also be used as I2C and SPI
Analogue inputs: 1 x 1.0V max
Size: 51 x 23 x 8 mm (without headers)
Featherweight: 9.7g
Charging: Built in 100mA LiPoly charger with charging status indicator light

Lipoly battery and USB cable not included.

Adafruit Feather HUZZAH Development Boards