Expansion packs or Addons came with a lot more high quality content with many more hours of actual gameplay content on a separate CD/DVD. The CD/DVD can be sold later.
Since 2006, a DLC is usually a minor content like a horse decoration, a car, a few short missions worth 1-2 hours gameplay, etc. DLCs come also with DRM, you cannot sell it later. (Also used in conjunction as pre-order bonus on e.g. Amazon or Steam) (Some still release Addons, some call Addons DLCs, but generally it is the way as described above.)