There are a couple important differences between Arduino and MicroPython. The first is that Arduino is an entire 'ecosystem' with the Arduino IDE (i.e. the desktop application you use to write and upload sketches), the Arduino programming language (based on C/C++), and Arduino hardware like the Arduino Uno R3 board.

MicroPython is only a programming language interpreter and does not include an editor. Some MicroPython boards support a web-based code prompt/editor, but with most MicroPython boards you'll write code in your desired text editor and then use small tools to upload and run the code on a board.

If you're coming to MicroPython or CircuitPython from a language that has a full IDE/editing environment like Arduino you might want some tips on how to configure and use a text editor and other tools for MicroPython/CircuitPython development. See these videos below for a deep dive into setting and using MicroPython/CircuitPython on different platforms:

The second important difference is that the MicroPython language is interpreted instead of being compiled into code the CPU can run directly like with the Arduino programming language. Interpreted means when MicroPython code runs it has to do a little more work to convert from MicroPython code to instructions the CPU understands.

A major advantage of interpreted code is that it can be much cleaner and simpler compared to languages that compile directly to CPU instructions. You can even write and run interpreted code like MicroPython directly on a board without any compiling or uploading--something that's impossible with Arduino!

One disadvantage of interpreted code and MicroPython vs. Arduino is that there's less performance and sometimes more memory usage when interpreting code. A function or sketch written in Arduino will run as fast as possible on a board's CPU whereas similar code in MicroPython will be a little slower because it has to interpret every instruction and convert it to CPU code. In practice this performance hit is rarely an issue for the kinds of projects you might create with Arduino. If you do run into performance or memory problems MicroPython allows you to write code in C/C++ or even the board's native CPU assembly instructions for maximum perfomance.

Ultimately there's no simple answer for the choice between MicroPython and Arduino. Each has strengths and weaknesses that should be considered for your own projects. Don't be afraid to try MicroPython--for some boards like the ESP8266 you can actually use either MicroPython or Arduino and pick the best one for your needs.