For a more complete explanation on this topic, I recommend reading this Educause article, “The Section 508 Refresh and What It Means for Higher Education.”
In a nutshell, an accommodation is a specific change that is made to allow a student to participate in a course. For example, an accommodation could be providing audio files of a PDF article that was required reading for the class. Or, and another example would be a student who is allowed a longer time period for taking their final exam than the typical exam period.
Accessibility is a change in practices that make it easier for everyone to participate in a course in the various ways that they learn. It may mean doing a pass at OCR on all articles assigned for a course, so students can either read or listen to the articles. It may mean ensuring that no course materials are color-coded, because this can be a barrier for anyone with complete or partial color-blindness. Or, it may mean taking videos of all class sessions and adding captioning so students may re-watch them on their own time to review missed concepts.
So while it is important and lawful to ensure that appropriate accommodations happen in the classroom to meet the needs of individual students, there are also many simple ways to incorporate accessibility into all of our teaching practices. This, I highly recommend that all faculty attend the upcoming IETL’s January Conference on Teaching, which will focus on the principles of Universal Design (note: registration is required), which include a variety of teaching practices to make learning more accessible for everyone.