Understanding accessibility requires an awareness of the special needs of multiple user groups, including people with disabilities and mature users with age-related disabilities. A person with a disability may encounter one or more barriers that can be eliminated or minimized by the software or Web developer, the assistive technology, or the underlying operating system software and hardware platform.

The four main categories of disabilities are visual, hearing, mobility, and cognitive.

Visual

People with visual disabilities are individuals who are blind, have low vision, or have color blindness. People who are blind need text equivalents for images, because their assistive screen reader technology cannot obtain information from images. A person who has a visual disability will not find a mouse useful because it requires hand and eye coordination. Instead, this person navigates the Web and software applications using only the keyboard. For example, the Tab key is used to move the focus to an item that needs to be selected. A screen reader then announces the item so the user knows where the focus is. The user then presses the Enter key instead of "clicking" the mouse button.

Low vision users need the assistance of a hardware or software magnifier to enlarge the text beyond simple font enlargement. Color blind and low vision users benefit from high contrast colors. When information is presented by color alone, a person who is color blind misses that information. Similarly, a user who has low vision might not detect the information if it is presented using any attribute by itself (e.g., contrast, depth, size, location, font, etc.). Use multiple attributes to convey information. For example, if both color and a fill pattern are used on different bars on a graph, they can be viewed in either color or black and white.

Hearing

People who are deaf or hard of hearing require visual representations of auditory information. Solutions for these disabilities include closed captioning, blinking error messages, and transcripts of the spoken audio. The primary concern is to ensure that audio output information is provided in a redundant equivalent visual form.

Mobility

People with mobility disabilities have physical impairments that substantially limit movement and fine motor controls, such as lifting, walking, and typing. Mobility impaired individuals experience difficulties in using the computer's input devices and in handling storage media. Solutions for persons with mobility disabilities include switches, latches, and controls that are easy to manipulate, as well as media that is easy to insert and remove. Software needs to be controlled without a mouse, or without a keyboard.

Additional solutions include alternate input capabilities, such as voice input or the ability to enter information at the user's own pace. For example, sequences of keystrokes can be typed, one at a time, rather than simultaneously as in Ctrl+Alt+Del. Many of these needs are supported by assistive technology, operating systems, and hardware platforms.

Cognitive

People with cognitive disabilities, such as dyslexia and short-term memory deficit, need more general solutions, which include providing a consistent design and using simplified language. For example, by using a template, a Web developer can reuse the same layout and design for each page, so a person with a cognitive disability can more easily navigate through a Web site. People with cognitive or learning disabilities can also benefit from redundant input, such as providing both an audio file and a transcript of a video. By simultaneously viewing the text and hearing it read aloud, they can take advantage of both auditory and visual skills to comprehend the material better.

Persons who are less familiar with the language of presentation also benefit from the same solutions that benefit those with cognitive disabilities. Cognitive solutions, especially simplified user interface, terminology, and examples, also benefit those who may experience educational or cultural disability. For example, people who are less familiar with computers.