What does megapixel mean? Are more megapixels always better? How many megapixels do i need? We explain it to you!

What does 300 dpi actually mean and how do I calculate 300 dpi? The term dpi comes from the English 'dots per inch' and denotes 300 dots (pixels) per inch (inch).

Why do so many keep dpi and ppi the same? When is it dpi and when is it ppi? And above all: what is the difference?