Test Plan: It's a overall plan for any for testing process in company how to start with testing. It defines with manpower requires, environment set up role & responsibility of an individual.
Test Strategy: It's a part of test plan. Test Strategy is different from different set of project. It defines the over testing approach along with what is the entry & exit criteria for each of it. Also it differs from project to project. An ecommerce project is different from insurance project. It is used by Project Managers, Test lead & Testers.