User experience (abbreviated as UX) is how a person feels when interfacing with a system. The system could be a website, a web application or desktop software and, in modern contexts, is generally denoted by some form of human-computer interaction (HCI).
Infographic: User Testing on Any Budget -- This is an EXCELLENT breakdown of what to test, how often to test, how many people to test with, and WHEN in your design/release process to test broken down by different budgets.
This infographic illustrates the amount of people in our population who are unable to access normal websites which don't take them into account. The infographic also lists easy fixes to these accessibility issues so web designers can quickly make a fix!