The rapid growth of RIA technology into the lives of every day people just a few years ago has carried both the usability and user experience industries to a new high in popularity. The success of software (particularly on the web) has driven both of these terms into our vernacular, and yet they are still often confused or thought to be synonymous. This post is meant to help those new to the field or unfamiliar with the intricacies of design to understand the differences between the terms.
Usability refers to the ease with which a user can accomplish his or her goals using any tool. Coming from the field of human factors engineer, usability has been applied to software in the fields of human computer interaction and includes many important ideas from psychology and statistics. Usability is fundamentally qualitative but involves the heavy application of quantitative data to identify areas of weakness and suggest improvements. The study of usability often focuses on performing extensive tests with large groups of individuals, sometimes involving in depth techniques like eye tracking to determine how users interact with interfaces and any areas in which they get lost. Highly usable interfaces are often lauded for being intuitive, simple or extremely learnable.
Somewhat in contrast, user experience refers to the way a user perceives his or her interaction with a system. User experience design encompasses both interaction design and visual design and seeks to promote an interface that is pleasing to the user. The study of user experience often focuses more on the psychological impact of interacting with the system than pure usability does, and user experience experts will spend their time performing both ethnographic and psycho-graphic research to construct their interfaces. User experience design is more qualitative than usability, though the two are not necessarily exclusive. For instance, often times user experience experts turn their designs over to usability experts to test and validate them in the field.
By far the easiest (and probably most over-cited, please forgive me) to distinguish these two comes on lauded usability expert Jakob Nielsen's homepage, useit.com. Jakob has dedicated a lifetime to the study of usability, and his website represents a page that is extremely easy to interact with. Everything is front and center, easily searchable, with important ideas stressed through bold text. Yet despite its high level of usability, the lack of interesting layout, design, or even typography makes the site rather boring and feel uninspired. I can accomplish my goals here easily, but I probably won't have much fun in the process.
On the flip side, let us consider interfaces which receive a high rating in user experience but perhaps a low one in usability. A good user experience is one in which the user is pleased with their interaction, but this says nothing about the user's ability to achieve their goals. While these two things are usually well aligned this is not always the case, and sometimes the most successful interfaces are those that can make difficult systems so enjoyable that even failing to achieve ones goals is still fun. My favorite example for this is, to the detriment of my online reputation and popularity with a large portion of the internet, Apple. Apple does an amazing job producing software that is fun to interact with but from a usability perspective much of it is atrocious. The iPhone is mired in confusing interfaces requiring a high number of clicks to achieve any goal, as anyone who has tried to use the "Maps" app for directions while driving can attest. If you need farther proof, just try and explain how to use iTunes or the app store to your grandmother. It's nearly impossible to understand without extensive time spent learning the interface and this is more or less the definition of low usability. Yet Apple's great reputation in interface design is not entirely without merit, as their attention to user experience is second to none. Failing on the iPhone is more enjoyable than succeeding on a Blackberry.
Perhaps the simplest way to distinguish the two would be to summarize them as "art" and "science", with usability representing science, though I caution you to use this callous and unrefined description sparingly and certainly not within ear shot of opinionated practitioners of either field, as it is sure to cause a (not wholly unjustified) rant. Yet the distinction is not without merit - user experience is often far more concerned with subjective artistry, and the emphasis on data crunching and numbers in true usability research is surely scientific. At the end of the day all that is really important is to understand the distinction between the two terms and to realize the massive importance of both in the design of good interfaces.
Credit for some of the ideas in this post goes to Timothy J. Wood, who let me listen in while he explained the difference between usability and user experience this morning.