Answer:
The difference between flat and eggshell is a very simple but important one.
Flat paint is not washable is should only be used on surfaces that do not get touched... ceilings are the best place to use flat paint.
Eggshell paint has the ability to be wiped, using a soft cloth and a very mild soap. This makes it perfect for the walls in a bedroom, a dining room or a living room. I have used eggshell throughout in homes that do not have children as adults tend not to get smudges and prints on their walls.