One of the most common things you will find on a job posting is an educational requirement. Most jobs require a high school degree; many require a college degree.
While you may not have thought twice about this before, consider the fact that not everyone has these degrees. If the company requires them, does that discriminate against potential workers who do not have them? Is that legal?
In most cases, educational requirements are legal, but not all. Generally, they're legal if the degree is related to the job and if it's necessary for the person to have that degree to do the job. The company isn't trying to discriminate against willing and able workers; it is just trying to find workers who actually have the skills and knowledge to do the job well.
When it becomes a problem is when the degree is not necessary and when it discriminates against people who cannot get it for reasons that are already protected. For instance, the Americans with Disabilities Act may define someone with a learning disability as being disabled, and their disability may make it impossible for them to graduate.
If they're then required to have a degree that they can't get when it's not necessary for the job that they want to do, that could be disability discrimination.
This is just one example, but it helps to show you how widespread discrimination can be. If you feel like you have been discriminated against on these grounds or any others, make sure you are well aware of the legal steps you can take.