This new Metro M0 Express board looks a whole lot like our original Metro 328, but with a huge upgrade. Instead of the ATmega328, this Metro features a ATSAMD21G18 chip, an ARM Cortex M0+. It’s our first Metro that is designed for use with CircuitPython! CircuitPython is our beginner-oriented flavor of MicroPython – and as the name hints at, its a small but full-featured version of the popular Python programming language specifically for use with circuitry and electronics.
Not only can you use CircuitPython, but the Metro M0 is also usable in the Arduino IDE. At the Metro M0’s heart is an ATSAMD21G18 ARM Cortex M0 processor, clocked at 48 MHz and at 3.3V logic, the same one used in the new Arduino Zero. This chip has a whopping 256K of FLASH (8x more than the Atmega328) and 32K of RAM (16x as much)! This chip comes with built in USB so it has USB-to-Serial program & debug capability built in with no need for an FTDI-like chip.
Key Features

ATSAMD21G18 @ 48MHz with 3.3V logic/power
256KB of FLASH + 32KB of RAM
2 MB SPI Flash chip
No EEPROM
32.768 KHz crystal for clock generation & RTC
3.3V regulator with 500mA peak current output
USB native support, comes with USB bootloader and serial port debugging
You also get tons of pins – 25 GPIO pins, 5 more than the Metro 328
Hardware Serial, hardware I2C, hardware SPI support
PWM outputs on almost all pins
6 x 12-bit analog inputs
1 x 10-bit analog output (DAC)
Built in NeoPixel on pin #40
Pin #13 red LED for general purpose blinking
Power on/off switch
4 mounting holes
We also include 4 rubber bumpers to keep it from slipping off your desk
Reset button
Adafruit Black PCB with gold plate on pads

Resources
Guide

FAQs:

Q: What is the METRO M0 Express for CircuitPython used for?
A: The METRO M0 Express for CircuitPython is a development board designed for use with CircuitPython. CircuitPython is a beginner-friendly version of Python that is specifically made for use with electronics and circuitry.
Q: What are the key features of the METRO M0 Express for CircuitPython?
A: The key features of the METRO M0 Express for CircuitPython include:

ATSAMD21G18 ARM Cortex M0+ processor at 48 MHz with 3.3V logic
256KB of FLASH memory and 32KB of RAM (which is 8x and 16x more than the Atmega328)
Built-in USB with USB-to-Serial program and debug capability
25 GPIO pins
Hardware Serial, I2C, and SPI support
PWM outputs on almost all pins
6 x 12-bit analog inputs and 1 x 10-bit analog output
Built-in NeoPixel on pin #40
Red LED for general purpose blinking
Power on/off switch
4 mounting holes and 4 rubber bumpers
Adafruit Black PCB with gold plate on pads

Q: What is the difference between the METRO M0 Express for CircuitPython and the Metro 328?
A: The main difference between the METRO M0 Express for CircuitPython and the Metro 328 is the processor. The METRO M0 Express uses an ATSAMD21G18 ARM Cortex M0+ processor, while the Metro 328 uses an ATmega328 processor. The ATSAMD21G18 is more powerful and has more features than the ATmega328. For example, the ATSAMD21G18 has 256KB of FLASH memory and 32KB of RAM, while the Atmega328 only has 32KB of FLASH memory and 2.5KB of RAM. Additionally, the METRO M0 Express is designed for use with CircuitPython, while the Metro 328 can be used with Arduino IDE.