Tactics:

Military tactics (Greek: Taktik?, the art of organizing an army) are the techniques for using weapons or military units in combination for engaging and defeating an enemy in battle.

Strategy:

A Strategy is a long term plan of action designed to achieve a particular goal, most often "winning." Strategy is differentiated from tactics or immediate actions with resources at hand by its nature of being extensively premeditated, and often practically rehearsed. Strategies are used to make the problem easier to understand and solve.

The word derives from the Greek word strat?gos, which derives from two words: stratos (army) and ago (ancient Greek for leading). Strat?gos referred to a 'military commander' during the age of Athenian Democracy.

Savielly Tartakower had it simplyfied:

"Tactics" describes what to do when something has to be done. "Strategy" describes what to do when nothing has to be done...

Rattler