What it is?
Usability addresses the relationship between tools and their users. In order for a tool to be effective, it must allow intended users to accomplish their tasks in the best way possible.

Why Usability is important?

• From the user's perspective usability is important because it can make the difference between performing a task accurately and completely or not, and enjoying the process or being frustrated.
• From the developer's perspective usability is important because it can mean the difference between the success and failure of a system.
• From a management point of view, software with poor usability can reduce the productivity of the workforce to a level of performance worse than without the system.

In all cases, lack of usability can cost time and effort, and can greatly determine the success or failure of a system. Given a choice, people will tend to buy systems that are user-friendly.

User Experience, User Interface & UI Design
User experience and interface design in the context of creating software represents an approach that puts the user, rather than the system, at the center of the process. This philosophy, called user-centered design, incorporates user concerns and advocacy from the beginning of the design process and dictates the needs of the user should be foremost in any design decisions.

The Importance of a Well-Designed Interface
The usability of your application's design is not just a philosophical nicety for your consideration; it is essential to the success of your application. The investment you make in the design of your application contributes not only to supporting and keeping your existing customers, but also to expanding your customer base. Therefore, your investment in the design of your application directly affects your current and future bottom line.

Fitts’s Law applied to Designing Web Pages
Interface design is difficult in part because everything requires interpretation. A design that works for one task or one user might not be appropriate for another. In other types of engineering, like architecture or bridge building, designers can always rely on laws of physics and gravity to make designs work. There is at least one immutable rule for interface design that we know about, and it's called Fitts's Law. It can be applied to software interfaces as well as Web site design because it involves the way people interact with mouse or other pointing devices. Most GUI platforms have built-in common controls designed with Fitts's Law in mind.

The basic idea in Fitts's Law is that any time a person uses a mouse to move the mouse-pointer, certain characteristics of objects on the screen make them easy or hard to click on. The farther the person has to move the mouse to get to an object, the more effort it will take to get to. The smaller the object is, the harder it will be to click on. It means that the easiest objects to locate and target are the ones closest to the mouse's current position and that have large target spaces.

Achieving High Level of Usability
The key principle for maximizing usability is to employ iterative design, which progressively refines the design through evaluation from the early stages of design. The evaluation steps enable the designers and developers to incorporate user and client feedback until the system reaches an acceptable level of usability.

The preferred method for ensuring usability is to test actual users on a working system. Achieving a high level of usability requires focusing design efforts on the intended end-user of the system. There are many ways to determine who the primary users are, how they work, and what tasks they must accomplish. However, clients' schedules and budgets can sometimes prevent this ideal approach. Some alternative methods include user testing on system prototypes, a usability inspection conducted by experts, and cognitive modeling.