est plan is a term and a deliverable. Test plan is a document that lists all the activities in a QA project, schedules them, defines the scope of the project, roles & responsibilities, risks, entry & exit criteria, test objective and anything else that you can think of.
This is also a deliverable and also a document at that. Test strategy outlines the testing approach and everything else that surrounds it. It is different from the test plan, in the sense that a Test strategy is only a sub set of the test plan. It is a hard core test document that is to an extent generic and static. There is also an argument about at what levels test strategy or plan is used- but I really do not see any discerning difference.