Essential course activities are tasks that are fundamental and necessary to complete the course: e.g., completing daily reading assignments, either independently or using an acceptable accommodation (e.g., books on tape).
Essential course activities include the amount of time spent on a specific task or duty: e.g., completing a paper by the expected due date listed on the syllabus. Instructors may accommodate students with disabilities by allowing them to use alternative testing procedures and formats for their assignments (e.g., for a student with a speech impairment, writing a paper rather than giving a presentation); however, students with disabilities can be held to the same work standards as their non-disabled peers (e.g., meeting assignment deadlines, etc.). These are not the only allowable accommodations under the law.
Essential course activities are also what the instructor believes to be an essential course activity: e.g., subscribing to a class email listserver and checking it regularly. The instructor can set his/her own class requirements as long as these can be verified as important (e.g., the student must have access to email because the large number of students in the class prohibit the instructor from photo-copying all handouts, so email document attachments are used to disseminate this information instead).
Finally, essential course activities are course requirements performed by past and current students in the course: e.g., using specific computer software, etc. However, instances where past students have expanded their class participation (i.e., gone beyond what the syllabus requires) are not considered "essential course activities" under the ADA.