A marketing strategy shows how to bring a product/service to market by exposing data and assumptions, explaining the positioning and the target and, finally, drafting the main strategies to reach and involve the audience.
Every strategy should be well researched: sufficient market data should support the assumptions that will be the base of the strategy.
Positioning is a crucial part of the whole marketing approach, and it helps to compare yours with the competition.
The key question is how you will use your resources (typically money): where, when and how will your message be seen.
Messaging is a core part of each strategy, as are all the creative strategies that guide the production of the marketing assets.
Finally this global strategy can be declined for specific channels that may require a strategy of their own. Social media, PR, sales channels, partners may require a specific marketing approach.