Open source hardware movement
3D printers, drones are here because of Arduino (and similar boards)
Arduino is a microcontroller on a board(dumber) made for electronic sketching.
They do only what you asked them to do NO OS
Raspberry Pi is a microprocessor on a board(smarter) made for programming.
Fully functional microcomputer
Somebody says as a rule of thumb: if you need to do more than two things use the RPi