PX (Pixels): Pixels are a fixed size. They don't change when the main font size or zoom level changes, so they're great when you need something to stay exactly the same size, like a small icon or a border.
REM (Root Em): This unit changes size based on the main font size of the webpage. If the main text size is adjusted (like when you zoom in), elements using rem will scale up or down to match. It helps make websites more flexible and responsive.
In short, REM adjusts with the overall size settings of the page, while PX stays the same no matter what.