It is differentiated from a retail business in that a store front, bricks and mortar, are not required. It is also differentiated from a catalog business in that a catalog is not required.
Catalogs are very expensive ways to advertise.
I feel these are 'must haves' for a successful e-commerce business. Your own domain name. You own web site. A product you can sell. Your own merchant program or the ability to accept secure payments on-line.