What is the logical difference between tailgaiting and piggybacking. and as per security concerns , which one is more risky for an organization?

Tailgating is the act of driving on a road too close to the vehicle in front, such that the distance between the two vehicles does not guarantee that stopping to avoid collision is possible. 

Biggybacking is while when an authorized person allows (intentionally or unintentionally) others to pass through a secure door when they enter.

The most risky for organization is Biggybacking because it allow unauthorized users to get physical access to restricted location which in most cases  followed with another security breach 

First of all let's define them both, e-commerce stands for