You may think that I get that question quite often. As a matter of fact, I can't remember the last time it was brought up by anyone in my work environment.
Wikipedia states that "User eXperience (UX) refers to the interaction of a person with a system, including their actions and perceptions, i.e., what the person wants to do, what they actually do and their feelings about using a product, a service, (…) or a combination of them."
Let's try to see how this definition would translate into e-commerce. Does User Experience cover features or look & feel ? Is it the satisfaction of a safe and sound transaction? Or could it also be this strong belief that you've found the product that is right for you, at the right cost?
The answer is: all of the above. "So this is all great, but do I really need User Experience ?", you ask?
Yes, you do (if you thought "no", please keep reading).
UX is the defining aspect of developing a good e-commerce solution. The truth is, User Experience cannot exist without other disciplines and vice-versa. And it's not just about making things look pretty.
Building a solution with a great UX requires a good collaboration between the business analyst, the developer leads (to name a few) and the UX designer. A collaborative and iterative approach is key. Simply put, the Business Analyst gathers the requirements, the Dev Lead validates them and the UX Designer brings them to life. Not only do we present and organize them a neat way, but we also design with your customer in mind.
Let's take a simple example.
Figure 1: Here is a shopping cart line item, presented in a wireframe.

Figure 2: The corresponding composite

The second figure shows the same content, divided into two concrete examples (product names have been removed to protect the innocent customer). A bold/negative color is used for negative actions that cannot be undone ("remove") and a positive/relaxing color is used on positive messages ("new release"). Everything else is in a dark, almost black grey on a light background for readability purposes.
The content is aligned with the table headers and empty spaces allow for faster scanning of the eyes, which helps the shopper proceed with the checkout as the next step. From a pure graphic prospective, the font, the bright colors combined with light shadows and gradients of grey give the cart a polished and professional look and feel.
And that's not it. The entire page is typically reviewed by the dev lead and the BA to validate the functionality and most importantly, by the Integration Lead who will then transform the composite into a Master Page and Cascading Style Sheets.
At Cactus, we like to think that technology supports, and content leads, but that User Experience rules.
This statement is not limited to B2C, its impact can be felt in B2B, government, education, and more. At the end of the day, there is always a customer on the other side of the screen who is looking for something: the right information, the right product, what the shipping options are, or how to return an order.
As a conclusion, I'd like to answer my own question. What is User Experience ? Well, I believe that a good user experience is like love: it is what happens when you are busy making other plans. Net, net, if you don't need to think about it, chances are, you've just had a good online experience.