A scenario is a story about, and general description of the context around, the tasks that will be performed in a user testing session. Though they aren't always necessary, if there is any back-story they need to understand the context scenarios can help participants to better put themselves in the shoes of the user they are representing.
Conversely, tasks are more specific and describe specific actions the user will perform within the system. It is important do describe the task in layman's terms, without using the name of any features or links you'd like a user to find--a good task description will make the task explicit without leading the user to any particular path of completing the task. It's also important to make sure that you don't use jargon in your user testing task descriptions, as it may confuse and/or intimidate test participants.
Keep in mind that it may be necessary to define unique tasks for different Personas or roles (e.g. student, instructor). This is helpful if your test participants would be have very different actions when they interact with the system. Ideally, the scenario should be written to describe a situation the participant would identify with in a real-life context. Though it may not be as effective, there are cases where it is possible to have the user 'pretend' that they are a different 'type' of user (e.g. a student pretending they are an instructor).
In some situations (such as when designing UI components) it may also be helpful to write scenarios which describe multiple areas of the system. It can make the test more cohesive if you have the participant work through several tasks that all naturally flow together in a single scenario. For instance, if you were trying to test a 'Pager' widget, you could test how it works in many different places in the Sakai learning management system. The items in parentheses in these scenario descriptions would not be included in a good task description, but are included her to illustrate the different contexts the scenarios describe.
An initial scenario could be from the perspective of a student who is working on the latest assignment for his Biology 101 class. He finds a course announcement from last week with a link to the assignment, and then finds a particular message (in the Forum/Discussion tool) to help him do the assignment. Before he submits the assignment, he reviews his grade for his past assignments (in the Gradebook) to try to figure out how good of a grade he needs on this assignment to be on track for an A in the course. To finish the assignment, he then needs to review some materials from another site from last semester which he's hidden, so he (goes to Worksite Setup and) finds the site in a somewhat but not too large list (perhaps 30 items, enough to initiate paging) of his sites.
Another scenario may describe actions using a 'Pager' from the viewpoint of an instructor. First he finds the student's assignment (in the Drop Box). He reads it and looks up the students' grades (in the Gradebook) to see how he's doing overall, then enters the grade for this assignment. As the student had a problem with a question that many other students had, he goes to the quiz (which he created in the Quiz & Survey tool) to find out how the question looked to students. Finally, the instructor finds a site for an Accessibility group on campus he's just found out about and is interested in and joins it (in Worksite Setup).
Examples of real-life user testing tasks & scenarios: