The accessibility movement encourages web sites to be built to allow people with disabilities to view them. For example, one accessibility standard is that all images have "alternate text" and "long descriptions" coded into the HTML.
This is useful for software that reads web pages out loud for blind people. Even if they cannot see images, the software can read the description of the image out loud.
The guidelines often used when determining whether a site is "accessible" are the Web Content Accessibility Guidelines. For more information on accessibility guidelines, visit the World Wide Web Accessibility Initiative website http://www.w3.org/WAI/.

Some of the many ways that a website should meet standards:

Table structure
Tables are built using relative sizing so that the page will resize to fit browser windows. All tables have a "summary" statement that describes what the table is being used for.
Cascading Style Sheets
Table background colors/patterns and bullet images are defined using Cascading Style Sheets within the theme (instead of hard-coding them, which FrontPage will do when themes are applied without CSS).
Font colors and sizes are also defined with CSS, which allows the page to degrade functionally even if someone does not have CSS viewing capability.
Images
Images within the page layout have "alt" and "longdesc" set in the HTML. (To edit the long description, you must go into HTML view.)

Web site accessibility, from a legal standpoint, is about giving people who are disabled as easy access to information through web sites as anyone else. Throughout the world there are approximately one in five people who fall under the legal definition of disabled.
In reality, the accessibility issue reaches far beyond those numbers since many people who do not fall under the legal definitions of disability may have difficulty using certain web sites. The fact is that most people need accessible websites at some time in their lives. The fact is that most people who need accessible websites do not consider themselves "disabled."
Accessibility is really an issue of inclusion, not disability. When we focus on the term "disabled" we compartmentalize and start talking statistics, which are variable and manipulable. Inclusion, sometimes also known as universal design, is about making the content and functionality of your website available to the widest possible audience.

The issue of website accessibility affects people who may:

• not be able to see, hear, move, or may not be able to process some types of information easily or at all
• have difficulty reading or comprehending text
• not have or be able to use a keyboard or mouse
• have a text-only screen, a small screen, or a slow Internet connection
• not speak or understand fluently the language in which the document is written
• be in a situation where their eyes, ears, or hands are busy or interfered with (e.g., driving to work, working in a loud environment, etc.)
• have an early version of a browser, a different browser entirely, a voice browser, or a different operating system.

Usability and accessibility are similar, but not the same. The ITTATC explains the difference this way:

Even if the technology is "accessible", there may still be serious usability problems that make it equally difficult for any person, disabled or non-disabled, to use it.

Usability focuses on how intuitive and easy it is for all people to use. Usable designs are consistent and simple to learn to use. Usability and accessibility often go hand-in-hand.

Accessibility is determined by how barrier free the technology is. Accessibility problems are those that make it more difficult for persons with disabilities to use an application or service than for a non-disabled person.