what is the diff b/n test plan ,test strategy,Test scenario and test case

Test Plan:
Test Plan is a document which describes the scope, approach and schedule of testing activities or you can also say that it contains "what we will test in software", "how we will test it", "when we test it as timing of testing" and "who will perform testing". It is the responsibility of test lead to develop this document.

Usually a test plan contains following things.
(1) Scope of testing
(2) Testing approach
(3) Time frame
(4) Objectives of testing
(5) What will be the environment
(6) What will be the deliverables
(7) Risk factors
(8) Features to be tested

Test strategy:
Test strategy is also a document and it is the responsibility of the project manager to develop it. It contains which techniques we will use and what will be the module to test. Here technique means type of testing. As different testing techniques can be applied to test software based on our goals. For example stress and load testing is applied on web based applications. We have to decide which testing approach we will use to test a module as following testing approaches can be used.

(1)Black Box Testing (2) White Box Testing (3) Ad-hoc testing (4) Acceptance Testing (5) Recovery testing (6) Sanity testing (7) Smoke testing (8) Regression testing (9) End to End Testing (10) System Testing (11) Functional Testing (12) Unit Testing (13) Alpha Testing
(14) Beta Testing (15) Exploratory Testing etc.

Test Scenario : A set of test cases that ensure that the business process flows are tested from end to end. They may be independent tests or a series of tests that follow each other, each dependent on the output of the previous one.

A test scenario specifies "What to test?"

Test Case : Is a document which describes the input, Action/Event and expected response to determine whether the particular functionality of the application is working fine.

A test case specifies "How to test?"

Order of STLC:

Test Strategy, Test Plan, Test Scenario, Test Cases.