There isn’t one particular aspect that defines the difference between b2c and b2b commerce. However, usually there are many differences. For example in b2b the person that buys is usually not the person that pays for the order.
Speaking of the payment: in b2b it is common to have specific pricing for each buyer, based on the volume or demand. Sometimes the assortment is customer-specific as well.
Another essential need for b2b ecommerce is a method to connect to other systems, such as an ERP, PIM, CRM or stock software.