There are a number of terms used by schools, colleges, and organizations such as the College Board for the steps they take to make classes, tests, and other aspects of education accessible to students with disabilities. We've written before about the right to accommodations in the classroom and other settings, but it should be noted that there is a difference between the common usage and the technical language of the applicable laws.
The Individuals with Disabilities Education Act (IDEA) doesn't specifically define the terms accommodations or modifications. The Americans with Disabilities Act and Section 504 of the Rehabilitation Act, which are the basis for most services provided to students after high school refer to academic adjustments and modifications, auxiliary aids and services [34 CFR 104.44] to describe those things that are generally called accommodations. These would be those things that enable individuals with disabilities to access something – a test, an assignment, a lecture – as effectively as they would have if they did not have a disability. Sometimes this is referred to as “leveling the playing field”. It does NOT change the work or the test; it just makes it accessible. Additional time for exams, a large print book, or someone to take notes for a student would all be examples of this.
The common use of modifications refers to a change in the material being taught or tested to make it easier for the individual with a disability to comprehend or master. This would happen, for example, when individuals with cognitive disabilities are taught different materials than their classmates. This would be a modification to the curriculum.
It is important to keep in mind that students in elementary and high school are entitled to a public education whether or not they are able to keep up with the standard curriculum. For some of these students it may be necessary to modify classroom material or tests to enable them to learn at a level they can comprehend. It is not unusual to see modifications of the curriculum for elementary or secondary students with complex learning difficulties.
However, once a student graduates from high school (or reaches age 21 if he or she hasn't graduated with a regular diploma) the right to a public education comes to an end. Colleges and other post-secondary programs are not required to make changes to the course work or tests if students cannot manage the standard curriculum. If you cannot do the work, you will not be able to continue at the school or program.
What is required in college and post secondary programs is that the student be provided with access to the materials being taught and tested and, in fact, to all the campus facilities.
There is a good explanation of these terms and how they work in academic settings in a publication by the U.S. Department of Education, Office of Civil Rights, called Transition of Students with Disabilities to Postsecondary Education. We think you'll find it helpful.