If you are asking this question you should start reading some tutorial, and check some basic documents for testing. I see that you asked a lot of questions which are very basic.
Here is a link to your question. Can you fallow the links in this page and read the software testing, system testing and all other questions that you asked today (unit etc.) or may be other questions planing to ask, before asking anymore questions. http://en.wikipedia.org/wiki/Test_scenario
Scenario can be a single test case or set of test cases. basically it is an end to end transaction to complete one task.
For example, Adding customer is one scenario/task, for which you may number of validations when you add a customer.
While adding customer, you may need to validate his/her enter details like first name, last name, email id, mandatory fields, etc for which you may have number of test cases. But adding a customer is single scenario.