First, we will explain what happens in a blade grinder.
A blade grinder is essentially two little blades that will spin at high velocity. In the same way a blender works. You dont have any ability to control the size of the grind size. What this essentially does is chop the bean like you were using a knife. It creates small splinters that don't brew your coffee that well.
A burr grinder is two burrs that essentially crush the beans into a size that you choose. Crushed coffee brews much better than coffee through a blade grinder.
So in summary a burr grinder is much better because they crush the coffee rather than cutting it up into small splinters. It also allows you to control the particle size.