Keyestudio Mega 2560 R3 is a microcontroller board based on the ATMEGA2560-16AU , fully compatible with ARDUINO MEGA 2560 REV3.
It has 54 digital input/output pins (of which 15 can be used as PWM outputs), 16 analog inputs, 4 UARTs (hardware serial ports), a 16 MHz crystal oscillator, a USB connection, a power jack, 2 ICSP headers, and a reset button.
It contains everything needed to support the microcontroller. With its bootloader, program can be downloaded directly with USB and you don’t need to use other external programmer.
Arduino UNO has 14 digital input/output pins (of which 6 can be used as PWM outputs), 6 analog inputs, a 16 MHz quartz crystal, a USB connection, a power jack, an ICSP header, and a reset button. It contains everything needed to support the microcontroller; simply connect it to a computer with a USB cable or power it with an AC-to-DC adapter or battery to get started