Difference between a User Story and a Use Case?

• What is a User Story? Simply put, written from the context of the user as a simple statement about their feature need. They should generally have this format. "As a -role-, I want -goal/desire- so that -benefit-"

• How is a User Story different than a Use Case? While a use case is highly structured and tells a story, the User Story sets the stage by stating the need. A User Story is the prelude to the use case by stating the need before the use case tells the story.
• How does the User Story fit into the process? User Stories are great as an activity in collecting and prioritizing the high level features. Getting this initial feedback from the customer is a simple way of trying to get all of their needs identified and prioritized. The User Stories will then morph themselves into the business requirements and use cases.

USE CASE Template- 

Use Case Element	Description
Use Case Number	ID to represent your use case
Application	What system or application does this pertain to
Use Case Name	The name of your use case, keep it short and sweet
Use Case Description	Elaborate more on the name, in paragraph form.
Primary Actor	Who is the main actor that this use case represents
Precondition	What preconditions must be met before this use case can start
Trigger	What event triggers this use case
Basic Flow	The basic flow should be the events of the use case when everything is perfect; there are no errors, no exceptions. This is the "happy day scenario". The exceptions will be handled in the "Alternate Flows" section.
Alternate Flows	The most significant alternatives and exceptions

A common myth with Agile Development is that you *must* use user stories, and not use cases. Like anything else in life, nothing is black and white -- being Agile is really about smaller iterations, learning and adapting to the market.

Source- http://www.gatherspace.com/static/use_case_example.html