These two words are often used interchangeably in many organisations and in business articles and books. However they have two very different connotations especially in terms of strategy.
If a strategy is implemented then it implies that it was not in the business previously i.e. either the business didn’t have a strategy or this is a new strategy. Also it implies that there will be a ‘start date’ and ‘end date of the implementation. That is at some point it will be in and working.
Implementing a strategy also often means that there is an implementation team. So I may or may not be involved. If I am not involved in the implementation then I wait for the team to come and tell me when it is ready.
Strategy is not a fixed item, it ebbs and flows and reacts to multiple influences. I would argue that it can never be ‘fully implemented’. Especially when many businesses ‘redo’ their strategy every year. At either calendar or financial year end the business gathers the leaders and they have their strategic planning session where they design the strategy and then spend X amount of time implementing it.
Implementation is better suited to change management or new IT / operating systems.
Execution on the other hand implies task level actions. There are several opportunities every day or every hour for an employee to execute.