TestPlan is controlled document which describes entire

testing process. It includes

Scoping of the test, Test deliverables, Test schedule,

Entry criteria, Exit criteria, Risks and contingencies and

also Test strategy.

Test Strategy or Test Methodolgy is a set of guide lines

that describes test design.

I think Test Plan is a high level document, which specifies 
What to test? How to test? When to test and who to test? 

Which contains
Scope&Objective:
Criteria:
Risk Analysis:
Test Procedure:
Test Environment:
Test deliverables:
 and Test Strategy is a document which specifies which 
method we have to fallow to complete this project with in 
time?

Test Strategy is nothing but it describes about that what 
kind of testings we are going to use in particular project. 
Test Strategy comes into Test Plan document.

Test plan is a high level document. It's road map of the 
entire testing activities. Test Plan covers about Summary 
of the project, scope, Features to be tested, Features not 
be tested, Test Strategy, Team Strength, Team member 
rights, Test Schedule, Exit & Entry Creteria, Assumption & 
Resumption, Contingencies. It's prepared by Team Lead.

But Test Strategy is a part of Test Plan document. It's 
telling about type of testing.

I wolud like to sahre more about on the test strategy.

Whole testing process have different testing level or 
heirarchy. Each tesl level has its objectives. To achevied 
that objective test strategy is built.

Suppose you are working on system testing. What steps 
should be taken to acheived the system testing objective 
will be considered as a test strategy.