A matter of scope and time. Tactics are what an individual unit or group of units (soldier, squad, ship, battalion, fleet) are doing (or will do) to survive or win an encounter. Strategy is how those movements and actions mesh with the overall campaign and the end goal of the team, army, nation. Combat units use tactics, commanders use strategy. A good commander makes his units aware of the overall strategic goals so that individual units (who are tactically engaged) can make decisions to contribute to the strategy of the whole.