How to u define these & whats there purpose?

[Legacy_Code]

hi,

could some one give the definitions of the following terms use in the "Script" that is used with the Use Case diagram:

1. Description
2. Pre-conditions
3. flow events
     1. Main flow
     2. Alternative flow
     3. exception flow
4.POst conditions

Note: PLease give their purposes and definitions in a simplified language  :-/

[Kevin Brennan]

Description: Overview of what the use case does and why.

Preconditions. Any fact that the system should assume to be true when the use case begins. This may include things like “ user must be logged in” or “ Item must exist in catalogue”.

Postconditions. Any fact that must be true when the use case is complete. May be divided into minimal guarantees (always true) and success guarantees (true if the goal of the use case is achieved).  The postconditions must be true for all possible flows through the use case.  

Flow of Events. Describes what the actor and the system do during the execution of the use case. The flow of events will be broken down into a basic flow (representing the shortest successful path) and a number of alternate flows that show more complex logic or error handling.