Web site usability and user interaction design focuses on customer insight; analysing a user's behaviour while performing tasks such as navigating through content, making an online purchase or following a designated call-to-action. It is essential that your site's visitor understands the "take-away value" of your marketing message, as well as offer positive reinforcement that your site supports their expectations and needs. The art and science of usability can help assure that your web site experience maximizes its potential and increases the potential for positive and frequent customer engagements.
A properly-designed usability study can actively provide measurable feedback on the success and failures of your web site's current (or proposed) user interface, prompting changes through experience design techniques to help ensure success.

Usability has five main components:
• Learnability – describes how easy it is for users to accomplish basic tasks
• Efficiency – once users have performed a task, how easy it is to perform the next task.
• Memorability – when users return to the site after a period of time, how easy is it perform previous tasks.
• Errors – how many errors are made by users, and how severe are the errors.
• Satisfaction – how pleasant the site is to use.

Usability and user interaction design are concerned with the behaviour of the web site or software in response to a user performing tasks as well as the "look and feel" of the interface. Interface designers and programmers should be concerned with understanding the tasks that the users will perform, then designing an intuitive user interface to support those tasks. You can validate those designs through usability testing methodology.
Usability testing is the truest test of website usability. It involves users being monitored while using this site, and their actions, comments, and reaction to the site recorded. This gives the designers a much more accurate indication of what and where changes need to be made (if any) to ensure that the site is as user friendly as possible. In the event that major changes are required for the website, this testing should be done before the final draft of the site is completed.

Usability testing is very beneficial for everyone involved with websites, designers, developers, administrators, the business providing the website, and of course, the users.

Usability testing can:
• Help discover the real needs and tasks of the user early in the design process
• Balance graphic design with functionality
• Provide tangible evidence for design recommendations
• Reduce costs by anticipating and eliminating potential user roadblocks
• Show significant cost savings beyond deployment of the product, through user productivity
• Provide a competitive advantage
• Quality Assurance (QA) of the entire site experience
• Increase user productivity
• Decrease user errors
• Decrease user acclimation time
• Increase accuracy of data input and data interpretation
• Decrease overall development and maintenance costs
• Decrease need for ongoing technical support
• Decrease customer support costs
• More follow-on business due to satisfied customers