Strategy, a word of military origin, refers to a plan of action designed to achieve a particular goal.
In military usage strategy is distinct from tactics, which are concerned with the conduct of an engagement, while strategy is concerned with how different engagements are linked. How a battle is fought is a matter of tactics: the terms and conditions that it is fought on and whether it should be fought at all is a matter of strategy.
For some, tactics are instinctive. Sometimes they can be taught. At its heart, tactics is a shifting amalgam of psychology, physics, and statistics.
Some practices have not changed since the dawn of warfare: ambushes, seeking and turning flanks, maintaining reconnaissance, creating and using obstacles and defences, etc. Using ground to best advantage has not changed much either. Heights, rivers, swamps, passes, choke points, and natural cover, can all be used in multiple ways. Before the nineteenth century, many military tactics were confined to battlefield concerns: how to maneuver units during combat in open terrain. Nowadays, specialized tactics exist for many situations, for example for securing a room in a building.