Resolution - the minimum distance between 2 points where they are still seen as separate points.
Magnification - the number of times larger an image produced by a microscope is compared to its real size.
total magnification = objective lens magnification X eyepiece lens magnification
The development of stains, better lenses and better light sources have improved light microscopy making cell structures / organelles be seen with more clarity and detail.
Electron microscopes have a greater magnification and resolution than light microscopes allowing much smaller things to be seen in more detail - have a better understanding.
The invention of the electron microscope allowed scientist to see sub-cellular structures too small to be seen with light microscopes e.g. mitochondria