the main difference is that paint is initially designed for bitmap images (working at the pixel level), whereas Photoshop is design to handle photos and can work at different levels (blur filters, saturation, etc.) or also at pixel level (event though it is not its primary purpose).
I personally use paint just for one thing. crop images in my clipboard (print screen) before pasting, but even that is easy in a software like photoshop.
For all the rest, I use gimp (open source alternative to Photoshop)