User experience design fully encompasses traditional Human-Computer Interaction (HCI) design and extends it by addressing all aspects of a product or service as perceived by users. HCI design addresses the interaction between a human and a computer. In addition, user experience design addresses the user's initial awareness, discovery, ordering, fulfillment, installation, service, support, upgrades, and end-of-life activities. HCI design constitutes a major portion of the activities performed by a user experience design team, so the following paragraphs provide an overview of HCI design followed by references to additional material about user experience design.
Human-Computer Interaction, or HCI, is the study, planning, and design of what happens when you and a computer work together. As its name implies, HCI consists of three parts: you the user, the computer itself, and the ways in which you work together.
The human perspective
HCI design teams must consider these factors in regard to users: what users expect and need, what physical abilities and limitations they may have, how their perceptual systems work, and what they find attractive and enjoyable when they use computers.
When humans interact with computers, they bring to the encounter a lifetime of experience. Even the first time we touch a computer, expectations learned in other areas of life can affect how we think a computer should work. For instance, because of our experience with other machines, we expect computers to provide immediate feedback when we press the on button. Elevators, automobiles, and other machines provide immediate auditory and visual cues that tell us that the machine is responding, and so we expect the same from computers. Without this feedback, we wonder if the computer is functioning properly.
Since users have various preferences, work environments, and physical capabilities, designers must also provide alternative ways for different users to communicate with their computers. Information can be exchanged by voice, keyboard, mouse, or other means.
Understanding how people's sensory systems (sight, hearing, touch) relay information is essential to designing a good product. For example, display layouts should accommodate the fact that people can be distracted by the smallest movement in the outer (peripheral) part of their visual fields, so only urgent conditions should be indicated by moving or blinking visuals.
And of course people like designs that hold their attention. Designers must decide how to make products attractive without distracting users from their tasks.
The computer's persona
In the natural world, most actions have obvious consequences. When you pick up your clothes from the cleaners', you see the clothes on their hangers, hear the rustling sound of their plastic sheaths, and feel their weight as you carry them. All these experiences serve as feedback confirming that you successfully completed your errand.
A computer carries on its business in a much less obvious way. The information a computer contains and the operations it performs are represented inside the computer in a form that we can't directly observe - binary digits encoded as two levels of electrical charge. What a computer displays or presents does not arise naturally from what it is doing inside. Any feedback the user might need must be explicitly planned out and programmed.
To make matters worse, computers don't even "think" as we do. They can remember amazingly large sets of instructions, but they have to be told every little thing in simple terms of "if this happens, do that" or "as long as this keeps happening, do that." And things we humans do almost automatically, such as jumping to conclusions or neglecting a trivial matter to take care of something more important, require even more extensive instructions to the computer.
So, given all these differences between humans and computers, how are we supposed to get along with them and get our work done? In other words, how can we interact with them effectively?
In order to come up with a product that's easy for people to use, software designers apply what they know about humans and computers, and consult with potential users of their products throughout the design process. When they know what their users want and need the product to do, they collaborate with programmers. Programmers know how to write instructions in languages that computers can understand. They also know what computers are capable of doing. The designers and programmers look for a reasonable balance between what can be programmed (written as computer instructions) within the necessary schedule and budget, and what would be ideal for the users. They have users try out any changes to make sure that the product is still easy, efficient, and pleasant to use.
As you see, designers and programmers play important roles in HCI, but users have the final say about the quality of the interactions.
User experience design
Whereas user experience design includes the human-computer interface, it is about designing the total user experience, which consists of all aspects of a product or service as perceived by users.
Here you can find more information about user experience design.