A Clan is a group of different units joined together to form an alliance which make up their Clan. These Clans have some Unique Races and Shared Races of units in them. The Unique Races of units found in most of the Clans sometimes make up their main theme to represent themselves with their Unique traits. For example: The Insect Race is most commonly included in Megacolony.
These themes may be used to form a deck. And they are usually themed around a certain ability or trait, such as preventing your opponent's units from Standing, Locking your opponent's units, calling units from the drop zone, drawing cards, etc.
Having units of the same clan is also especially important for Triggers. To use the effect of any Trigger, you must have at least one unit of the same clan as the trigger in the Vanguard or Rear-guard Circles. Many units have negative skills that punish the user for mixing clans such as Lord, and restrictions that prevent them from being used in decks focused on different clans such as requiring a Vanguard of the same clan. Because of these restrictions, it is generally recommended to build a deck from only one clan, however, it is not mandatory to do so. It is not necessarily a bad idea to mix clans in decks, in that case it is recommended not to include cards with skills that punish mixing clans. Originally, most Clans (outside of "main" ones like Kagerō and Shadow Paladin) tend to have only 4 cards (1 in each grade) and no Trigger Units in the set they are introduced.
Currently, Etranger is the only clan without enough confirmed support to build a full deck based around it.
List of Clans
United Sanctuary cards are represented by the color yellow.
Dragon Empire cards are represented by the color red.
Dark Zone cards are represented by the color purple.
Magallanica cards are represented by the color blue.
Zoo cards are represented by the color green.
Star Gate cards are represented by the color grey.
Here belong clans without a nation, some of these are just joke cards and illegal in official bushiroad tournaments.