So you know of Strikes. Once you gain access to these missions, you may replay them whenever you want and they will remain the same. As for the story... We don't know much. In the Alpha there was one story mission, which rather felt like a Strike, but with a much weaker boss. My assumption is there will be the Campaign, a set line of missions varying in type, from what we saw to missions that don't even have their own specified type, increasing in difficulty as you go.