An Arduino has a microcontroller. A microcontroller is a simple computer that can run one program at a time, over and over again.
A Raspberry Pi is a tiny general-purpose computer, usually with a Linux operating system, and has the ability to run multiple programs normally in Python.
"Raspberry Pi is a series of small single-board computers developed in the United Kingdom by the Raspberry Pi Foundation in association with Broadcom. The Raspberry Pi project originally leaned towards the promotion of teaching basic computer science in schools and in developing countries."