The ADA (American's with Disabilities Act) was signed into law in July of 1990. It is enforced by the Department of Justice, and it governs how public places should be accessible to people with disabilities. One key thing to remember is that it applies to all public places (office buildings, public spaces, restaurants, etc.)
Any American can file a claim that a building or space is not in compliance with the ADA- and they do. The Department of Justice is required to investigate all such claims. The Attorney General can also get involved in this area of compliance enforcement. Many times, and individual or a local code inspector are the initiators of these claims. Anyone can file a civil lawsuit that a building is not ADA compliant.
In the early 1990’s, there was a great deal of litigation as businesses and building owners were slow to make the required changes. This litigation dropped off in the late 90’s until around 2002. Around this time, individual states began hiring their own compliance officers tasked with ensuring compliance. The ADA statutes were updated again in March of 2012. Fines for non-compliance start at $55,000 for the first offense.
Also find out the most common question about ADA here.