Good question - where a RPi can basically run as a desktop computer, an Arduino is a lot simpler - and lower-level to work with. Arduinos can be used for small electronics projects but require programming to work with them. Where a RPi can be a multi-functional server/desktop computer - like running Pi-Hole, media server, etc - an Arduino would be programmed for a specific purpose, like a MIDI controller or to be the "brain" of a robot.