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?
Best Answer
The two terms are orthogonal - an expansion pack might be DLC, but that would imply two different facts:
DLC - "downloadable content" - is a very broad term for any additional parts of a game that you can obtain from any of the various online vendors. It's generally reserved for official content from the original publisher, to distinguish it from community content. That is, an additional module for Skyrim to add fancy armor for your horse would be a "mod" if a user did it, but "DLC" if Bethesda did it. DLC can also be very big; for Skyrim, both Dawnguard and Hearthfire are DLC and they make extensive changes to the game.
An "expansion pack" is a separate product that builds on top of an existing game to add significant new features. Expansion packs can usually be purchased at retail and installed from CD, and exist separately from their parent game. They are typically "mini games" in their own right, but rely on the content and engine of their original game for much of their content. In Skyrim, Dawnguard is obviously an "expansion pack" - it add new lands, new weapons, new skill trees, etc. I don't know if it's available for retail yet but I would be very surprised if there's not a Skyrim GOTY edition that includes the two expansion packs.
A better example might be Oblivion: there was a handful of DLC released, including horse armor and Knight of the Nine. Later, there was an actual expansion pack - Shivering Isles - that added an entire new land and new features into the game. (The line was blurried a bit when Bethesda packaged all of their DLC onto a CD for retail sale, but it was basically the same as downloading the DLC and burning the archives to to disk.)
With so much game delivery being online these days, and with official downloadable content getting bigger, the line between a "plug-in" and an "expansion pack" is blurring, and all of it would be considered "DLC".