tl;dr: The Raspberry Pi Pico is a new $4 microcontroller board with a custom new dual-core 133 MHz ARM Cortex-M0+ microprocessor, 2MB of built-in flash memory, 26 GPIO pins, an assortment of SPI, I2C, UART, ADC, PWM, and PIO channels.
It also has a few other party tricks, like edge castellations that make it easier to solder the Pico to other boards.
The Pico is powered by a new RP2040 chip—a brand new Raspberry-Pi-built ARM processor. And the best thing about this processor is the insanely-detailed Datasheet available on the Pico website that steps through every bit of the chip's architecture.
Video Review and 'Baby Safe Temperature' Project
I posted an entire video reviewing the Pico and demonstrating a MicroPython project. The video is embedded below: