Graphic shamelessly stole from What is User Experience?
Depends on who you talk to. I like to say that it boils down to how easily, effectively, and pleasurably one achieves ones goal's when using something.
User Experience Architects
"User Experience Architects" (UX Designers/Information Architects/Interaction Designers/UI Designers) provide user research, information architecture, interface and interaction design, and usability testing. We conduct primary research, often in the form of interviews, to understand the goals, attitudes, and behaviors of users of websites and applications. This understanding is expressed through personas, scenarios, mental models, and task flows. These deliverables are shared with clients and project team members and are used to inform strategy, prioritization, messaging framework, taxonomy, information architecture, and design (including visual design).
UXAs create site maps, user flows, and wireframes to define logical grouping of content and features, page elements, navigation systems, labeling, and interaction behavior. UXAs evaluate the usability of designs via heuristic evaluation, paper prototyping, and clickable/interactive studies. This work is collaborative with editorial, taxonomy, SEO and visual design.
The Value of User Research
In short, research ensures that knowing triumphs over guessing.