The user experience (UX) is the experiential, affective, meaningful and valuable aspects of human-computer interaction and product ownership, but also a person’s perceptions of the practical aspects such as utility, ease of use and efficiency of the system. Essentially, it's about how a person feels about using a product.
User experience is dynamic as it is constantly modified over time due to changing usage circumstances and changes to individual systems as well as the wider usage context in which they can be found. The end-user experience is about how the user interacts and experience the product.
Note: There's now an entire Stack Exchange site devoted to the subject of User Experience, so please consider whether your question would be better asked there.
Questions on Stack Overflow should be about the implementation of something that arises as a result of UX considerations. It could cover things like:
- AJAX updates of web pages to avoid page refreshes.
- Implementation of validation of and verification schemes
They should not be about whether design "A" is better than design "B" etc.
