What an and Chris said, really.
Arduino : a very cheap and basic and easy to learn microcontroller. It's really no different to what you'd find running a washing machine, for example. The only difference is that it's been designed for easy learning, easy programming, and super easy to connect sensors etc. to. Typically run at a few Mhz (8 I think, but some are 16) and has a few kb of ram.
Pi: it's an actual computer with a cpu and gpu and all that, and runs Linux (and windows 10 at somepoint). Runs at around 1ghz and has 1gb of ram or more. Only difference between a Pi and a PC us that it's the size of a credit card and a number of the cpu pins are available to connect external sensors etc. to.