Since user experience design is a relatively new field, I often have to explain what it is to various business partners. I've done several presentations on the topic.
User experience design centers around the needs of the user. To paraphrase Tolstoy, all the sites that make users happy are alike, but each poorly designed website makes the visitors unhappy in its own way. The user friendly sites share these traits:
• Effective websites lets the users accomplish specific tasks that they want and/or need.
• Efficient websites enable completing the task within a reasonable amount of time with reasonable effort.
• Engaging sites make the completion of the task a pleasant, positive experience. A site that is elegantly designed will make the whole process look more pleasant.
• Error tolerant sites help the users avoid mistakes and easily recover from mistakes.
• Easy to learn websites contain organization, workflow, icons, and other elements that are based on what the users already know.