Accessibility guidelines are primarily developed for people with either visual, hearing or physical impairments and disabilities. Here are basic descriptions of some ways people with disabilities use the internet:

• People with visual impairments might use a screen reader. This is a software that will read out loud the text of the web page. They might also use a text browser and need to make the text bigger. Just think about the elderly that have weaker eyesight.
• People that have hearing impairments won’t be able to hear music or audio.  Captions or transcripts are the only ways to not alienate them.
• People with physical disabilities like reduced motor skills use assistive technologies to help them navigate through the site.
• We also need to keep in mind people with cognitive disabilities. They’ll need simple language and consistency throughout the website. This is also valid for first time web users.

There’s lots more to consider, for more read: W3C’s document on how people with disabilities use the web.