UNITS are beings that you control and play with in the game. Units are clones of each other and are generated, hired or bought in potentially infinite numbers.
Common characteristics:
Die permanently
Are purchased/hired in large quantities
Can be upgraded
Have attributes and/or abilities
Examples:
Troops that you train from barracks or other buildings (e.g. Barbarians in Clash of Clans)
Towers in tower defense games, if there are no limitations to the amount of towers to be built per stage (e.g. plants in Plants vs. Zombies 2)
NOTE
A single entity in a game can be classified as only one of the following: CHARACTERS, UNITS, or BUILDINGS! For example, UNITS are not CHARACTERS, as UNITS can die permanently and can be trained/created from e.g. barracks in large numbers over and over again if you just have enough resources.