Someone else mentioned Mintzberg, well worth a read. It will shatter many folk theories of what strategy is.

Having said that I'll give a vague answer.

Strategy is consciously deciding on one policy while sacrificing another. If we play a high line, we can't low block around the penalty area. If we press like mad, we can't conserve our energy. If we play the tall guy up front we are committing to crossing the ball, not passing it on the ground.

So basically it's a question of what we're doing and what we are choosing not to do, aka tradeoffs.

The reason things get vague is that we often get mixed up about what is the goal and what is the policy to get there.