Test Strategy is part of the Test Plans. Test Plans describes who will do what? Describes roles and responsibilities of test team, resources required any risks associated, major functions to be tested. Test Strategy is a very important part of the test plan. When requirements analysis phase is finished and requirements are somewhat mature then testing team starts preparing test plans, how to carry on testing and what strategy to be followed.
In very simple precise terms there are two basic testing strategies:
To test the software in its entirety, once the completed package is available; otherwise know as “big bang testing”
To test the software piecemeal, in modules, as they are completed (unit test); then to test groups of tested modules integrated with newly completed modules (integration tests). This process continues until all the package modules have been tested. Once this phase is completed, the entire package is tested as a whole (system test). This testing strategy is usually termed “Incremental Testing”
Incremental Testing is further divided into two classes Bottom-up Testing Strategy and Top-Down Testing Strategy.