ADA stands for Americans with Disabilities Act, a law passed in 1990 that establishes protections for people with disabilities. This means disabled people must have access to employment opportunities, transportation, public accommodations, etc. What exactly ADA compliance entails depends on the context; in civil construction and architecture, for example, this means your public facilities must be accessible to disabled people. In particular, companies have been putting much effort into improving the accessibility of their websites in the past few years. That's because the internet is now understood as a public space and therefore subject to the ADA, though the law doesn't yet have specific standards for web development. Instead, the ADA encourages the use of the Web Content Accessibility Guidelines (WCAG) 2.0 Level AA guidelines from the W3 Consortium. You can find many tools to evaluate compliance to the guidelines; a list of which can be consulted here.