On the east coast Italian restaurants (or pizza shops) sometimes offer both Strombolis and Calzones. Sometimes it seems that Calzones are a 'type' of Stromboli, because the menu has a single Calzone but a list of different Strombolis. But I doubt that's an accurate assessment.  So, what's the difference between the two? I'd be interested in the technical reasons as well as whey they're listed separately on menus (if the reasoning is different).   Also, I give the east coast location because I've heard out west a Stromboli may be thought of as a kind of hot sandwich (basically an Italian roll hollowed out a bit and filled). That's not the Stromboli or Calzone I'm familiar with - but if the definitions change drastically based on geographic area I'm interested in that as well. 
In my experience the primary difference is that Calzones have Ricotta (and possibly Mozzarella) and Strombolis only have Mozzarella.  In the Philadelphia area, both are folded over pizzas, basically.  Growing up, my mother made Stromboli and it was rolled, not folded.  I rarely see that in a pizza shop these days, but I do prefer that in a Stromboli.  Edit: Just read systempuntoout's link to Wikipedia and it confirmed something that I was going to post and then deleted.  When I was growing up, Strombolis were made from bread dough and Calzones were made from pizza dough; nowadays all of the pizza shops just use pizza dough.  I remember bread stores as being the primary place to get Strombolis growing up because they were the ones that made the dough! 