The Arduino Uno and Mega are both microcontroller boards based on the same ATmega328P microcontroller chip. The main difference between them is the number of input/output pins (I/O pins) they have. The Uno has 14 I/O pins, while the Mega has a whopping 54 I/O pins. This means that the Mega can be used to control more devices than the Uno.
For example, the Uno can be used to control a single motor, while the Mega can be used to control multiple motors at the same time. The Uno also has fewer memory and storage options than the Mega, making it less suitable for complex projects.