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.
Originally, most cards had restrictions that prevent players from adding cards from other clans (such as Lord or requiring a Vanguard from the same clan to use their skills) to discourage clan mixing, however, mixing clans was allowed and wasn't necessarily a bad idea in some cases. As of Season 4, mixing Clans is no longer allowed (except for some exceptions) so these restrictions were removed as they are no longer necessary.
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.