Bungie already talked about this some time ago, things may have changed. However, if memory serves there are going to be 3 core games, ie Destiny and 2 full sequels, coming out with a major expansion in between each release. With a sprinkle of smaller DLC content to tide us over here and there.
Think of it like a book series. The games are like books, each a complete story with a distinct beginning and end, that also tell the larger over arching story of the series as a whole. The expansions are like a side story that comes out to fill in some of the blanks and keep readers interested while the next book is being written.
Something like that, or at least that's the last I've heard on it. Although given the goal of a 10 year series, this mixed release strategy has the best chance of carrying the series to the end.