Here's some information from the National Center on Educational Outcomes (NCEO) about accommodations:
Assessment accommodations are changes in testing materials or procedures that enable students to participate in assessments in a way that assesses abilities rather than disabilities. Without accommodations, the assessment may not accurately measure the student's knowledge and skills.
Assessment accommodations are generally grouped into the following categories:
Presentation (e.g., repeat directions, read aloud, large print, braille, etc.)
Equipment and material (e.g., calculator, amplification equipment, manipulatives, etc.)
Response (e.g., mark answers in book, scribe records response, point, etc.)
Setting (e.g., study carrel, student's home, separate room, etc.)
Timing/Scheduling (e.g., extended time, frequent breaks, etc.)
Some test changes affect item or test validity. These test changes are often called modifications, or non-standard accommodations.