Usually when creating an information project, you would start by describing the people who are likely to use your product, and what they need to get out of your product. These are called user personas and are very helpful in generating a common understanding with everyone on the project about the users and their goals. To make the personas more concrete, you should include photos of people who look like the users you describe. (Stock photos are fine as long as they are casual, and always make sure you have permission to use the photo.) Photos put a human touch on the personas.
The next step after personas is to describe actions that your users would take to accomplish their goals. These are called user scenarios. Scenarios can be very detailed, or they can be left a high level - but they always describe how your users do their work. There's nothing like being with your users when they try to accomplish a task, so you can watch them first-hand, but a good "second best" is to capture that behavior in a user scenario.
In usability testing, you start not with the user personas, but with the scenarios. These usability scenarios are usually described at a high level, as you do not want to burden your usability tester with a preconceived notion for how they should accomplish a task, simply provide a description of the task or tasks they need to perform using the information product.
In my example of usability testing, I will ask usability testers to complete several tasks, described as scenarios. The scenarios will start with a brief context, then a textual description of the task the tester is asked to perform. In the usability testing field, these are elaborated scenarios.
I'll describe the scenarios in other posts here.