It's a computer; you can do all the usual computery things with them. Main differences between a Pi and a typical PC:
- It's cheap.
- It's slow and limited in RAM (especially an older model, which will be barely usable for normal desktop applications like web browsing). The hardware is optimised for cost in a way that makes network and USB performance underwhelming. Using an SD card as a system disk also makes it fairly sluggish.
- It can't run Microsoft Windows, but is well supported by several Linux OSes, and some geekier alternatives.
- It's got lots of lovely IO that you can use for connecting sensors, blinkenlights, robotics projects,
bonkers art projects, etc.
- It doesn't use very much electricity, which means you can plausibly run it from batteries for electronics project stuff, and also makes it a reasonable choice for lightweight server applications.
- It's cheap, consistent and readily available, which means that lots of people have used it as a basis for the sort of things you can do with an embedded Linux system. Think media players, emulating classic gaming systems, weather stations, time servers, etc, etc.
It was conceived as cheap hardware for kids to learn to program on. Think of it as the BBC Micro for the 2000s (at ZX Spectrum prices), and you've got the right sort of idea.