In the default TFS setup there are three work item types: scenario, task and bug. That last one is quite straightforward, and task also: it\'s a specific job for a team member t
If by "default TFS setup" you mean the "MSF for Agile Software Development" project template, then a scenario is defined as follows:
A scenario is a type of work item, recording a single path of user interaction through the system. As the persona attempts to reach a goal, the scenario records the specific steps that they will take in attempting to reach that goal. Some scenarios will record a successful path; others will record an unsuccessful one. When writing scenarios, be specific as there are many possible paths.
To get a bit more info on this, have a good look at the "Documents/Process Guidance" folder under the project in team explorer - it explains the recommended process fairly well