In terms of access rights, there is no difference between being “Assigned” or “Reviewer” on a task.
The difference between one and the other pertains to work allocation. When a user is “Assigned” on a task, he/she will share the Estimated Hours allocated on the task with the other “Assigned” users, whereas a “Reviewer” will not be allocated any hours on the task. This difference is obvious in the task assignment grid.
Generally speaking, an “Assigned User” will perform the work whereas a “Reviewer” will only oversee the work. Assigning a user (or oneself) as a “Reviewer” on a task is a mechanism that allows that individual to receive e-mail notifications for updates on the task, thus facilitating its “review”.