The base game is usually the first complete released piece of content. It is also referred to as 'vanilla', as in the unflavoured or basic flavoured version. In some instances you can already get versions which include additional or exclusive content (presented with cool names like Gold Edition, or Collector's Edition, the latter of which often features additional physical trinkets), or will include future content (which is often available separately as well). 

All DLC also often comes in the form of a so-called Season Pass.

Newer releases can include more of the content that has been released in the mean time, like all DLC. For example, when a game has been selected as Game of the Year, it is usually rereleased as a Game of the Year or GOTY edition, and as far as I know, always includes all DLC/the Season Pass. 

DLC is an abbreviation for **D**own**L**oadable **C**ontent.