Test strategy is a high level document which defines the approach for software testing. It is basically derived from the Business Requirement document. Test strategy is developed by project manager or business analyst. It is kind of static document which sets the standards for testing so not updated often.
Test plan is derived from SRS (Software Requirement Specification) which is prepared by test lead or manager. The main goal of test plan is to include all the details related to testing such as what to test, when to test, how to test and who will be the tester. Test plan is often not updated but if there is some new feature or change is introduced then it has to be updated accordingly.
Having a detailed and customized test strategy and plan created for you is a great first step in creating a streamlined product development process that takes quality into consideration from the very beginning. A process that catches bugs early, reduces development time, lowers cost to company and gets you faster to market with a better quality product.