asins creed 2 storyline?

im up to when you move to this new city with leonardo and have just stolen the boat and the clothes from the 3 chests to asinate some guy. but i dont really have a clue whats going on in the storyline and no idea what the plot is ive kinda just been playing blindly listening to what the objectives are. i understood the first bit when we kill the people responsible for his father and brothers death but thats about it. can you explain both storylines about Ezio and Desmond and how they link???