Task is a one of the process steps. To complete the process all process' tasks should be completed.
As a user you may have different permissions to access the exact task. It is based on your role in exact process (you can manage the process execution, you may be a assignee of the task or candidate assignee, or just a user, who mentioned in task comments)
If you are a Process manager you will have full access to the task but can
access it only via process view page in "My processes" and "All processes" list on Processes screen.
On process view you can see a list of all tasks in a process:
Process manager can:
View all task details, data form, comments, history.
Edit and delete the task on Process edit page
Change task due date
Set or change task assignee
Complete and Reopen the task
Complete the task on behalf of task assignee
If you are a Task Assignee you can view it in "My tasks" list on Tasks screen.
"My tasks" and "Available tasks" lists:
Task Assignee can:
View all task details, data form, task comments, history.
Change task due date
Unassign the task from yourself
Complete and Reopen the task
If you are a Candidate Assignee (means you can assign the task to yourself and work on it) you can view it in "Available tasks" list on Tasks screen.
Candidate Assignee can:
View all task details: task description, data form, task comments and history .
Assign the task to herself or another user.
Change task due date.
If you mentioned in task's comments and are not participated in process as process manager, starter or assignee or other role you can view the task by click on task link in email notification.
If you are an Administrator you can access the task only via process view page in "All processes" list on Processes screen.
Administrator has the same permissions to task as Process manager.