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 exceptional cases) so these restrictions in later cards were removed as they are no longer necessary.
In Cardfight!! Vanguard G, top level fighters are honored with the title of "clan leader."
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.
Other than the above three clans, Etranger also belonged to Star Gate in terms of regulation, however, it was used as a sole purpose of cameo cards. And do not have trigger unit to build a single clan deck. Which essentially making all but one of them in illegal in Clan Fight Regulations.
Cray Elemental's card belonged to all clans and nations, which appears as a part of ability. They are represented in rainbow colour.
These two clan appeared as non-tournament joke cards.