Many years back, before the term "downloadable content" was born, when game developers released new content and functionality for their existing games they called it an "expansion pack". This was generally sold in a box from a store, just like the original game, and required the original game to play - very much like downloadable content.
Now, the term "expansion pack" is not something that I see very often, if at all. Instead, everything seems to be called "downloadable content", is very rarely available in a box from a store, and is normally distributed over the internet through some form of digital library system like Steam, or from the developer directly.
I feel this is more than simply "what marketing chose to call it", Blizzard for example never call their new WoW content "downloadable content", even though each of them is automatically downloaded via their launcher - but instead always call them "expansion packs".
Case in point, during some discussion about future content for Skyrim (from some time ago now, since Skyrim has been out for more than a year already), Todd Howard at Bethesda is quoted stating the following;
"For Fallout 3 we did five DLCs," Howard told me during an interview last week at the DICE Summit. "That was a very aggressive path for us. Our plan now is to take more time, to have more meat on them [for Skyrim]. They'll feel closer to an expansion pack."
bold = my emphasis
What is the difference between an "expansion pack" and "downloadable content", or are they essentially the same thing with a decade of evolution on what we call them?