The defined role for a task can be either a Task Audience or a Task Owner or, in the case of a parallel workflow task, both.
The roles of a Task Audience and Task Owner work differently for a standard task and a parallel task in a workflow.
In a standard task, a Task Owner defines a role that contains a single user. This user will be assigned the job ticket. A Task Audience defines a role that may contain any number of users. The job will be posted to these users. They will be the audience for the job, but it is not assigned to them.
Generally, for non-parallel workflow tasks, you will specify either the Task Owner or Task Audience, but not both. Specify a Task Owner if there is a specific user that should work on this job and it will be assigned directly to them. Specify a Task Audience if there is a pool of users that need to know about this job. The job is posted to the pool of users. Any member of this pool can accept the job, in which case it is then assigned to them.
However, in a parallel workflow task, each member of the Task Audience is assigned to the task. The Task Owner monitors the progress of the individual parallel workflow tickets. So for a parallel workflow you will specify both a Task Owner and a Task Audience. The status of an object cannot change until all of the parallel job tickets have been completed.