The Raspberry Pi is a phenomenon. The original was the size of a credit card (version A), now it is a bit bigger than that but not by much (version B). There is a smaller version called a zero which you can buy for £5 and it is a fully fledged computer. They even gave it away free with an edition of their magazine MagPi. The first time that a computer has been given away free with a magazine.
The Raspberry Pi is now the best selling computer in the UK. You simply connect a keyboard, mouse and monitor, power it up and away you go. Its operating system is on a SD card and uses a form of Linux (which is open source and free).
It was created for the purpose of teaching people how to code. There are lots of things you can attach (called HATs) to measure or control different things. Python is pre-installed with the software and is the main coding language that it uses. It isn’t fast or powerful like a modern laptop or PC but it is cheap and does most things a computer is meant to do.