As with many games, Unit cards in Eternal can have one or more subtypes. This categorises the particular Unit and can be used by other cards for purposes of targeting, buffing, etc.
Currently (card set 1) all units have a subtype but the game doesn't make a lot of use of them. Apart from the Strangers and Clockroaches only a few cards such as Avisaur Patriarch and Wump, Party Starter are subtype dependant. This is likely to change as future cards are released, creating more opportunities for building themed decks.
Here's a full list of the subtypes in the game at the time of writing:
List Of Unit Subtypes
This page written for Eternal version 1.18, Card Set 1 Last updated: 29-03-2017