This is the Adafruit Feather M0 Bluefruit LE - an 'all-in-one' Arduino-compatible + Bluetooth Low Energy with built in USB and battery charging. Bluetooth Low Energy is a hot, low-power, 2.4GHz spectrum wireless protocol. In particular, it's the only wireless protocol that you can use with iOS without needing special certification, and it's supported by all modern smart phones. This makes it excellent for use in portable projects that will make use of an iOS or Android phone or tablet. It also is supported in Mac OS X and Windows 8+. To make it easy to use for portable projects, 3.7V Lithium polymer battery connector and built in battery charging means you don't need a battery, it will run just fine straight from the micro USB connector. But, if you do have a battery, you can take it on the go, then plug in the USB to recharge. The Feather will automatically switch over to USB power when its available. We also tied the battery thru a divider to an analog pin, so you can measure and monitor the battery voltage to detect when you need a recharge. Item Specifics No EEPROM 3.3V regulator with 500mA peak current output USB bootloader and serial port debugging 20 GPIO pins Hardware Serial, hardware I2C, hardware SPI support 8 x PWM pins 10 x analog inputs 1 x analog output Built in 100mA lipoly charger with charging status indicator LED Pin #13 red LED (general purpose blinking) Power/enable pin 4 mounting holes Reset button Size: 2" x 0.9" x 0.28 Weight: 5.7g