"Clans" (see the Wikipedia article) are groups of players that play an online game together. They are more or less "stable" teams of people that train together and play matches against other clans.
Usually, players from a specific clan add a distinctive text ("tag" or "signature") at the begining of their name. For example, given a clan named "My Own OA Clan", the clan tag could be, for example, [MOOAC]. Thus, the nicknames of two players called "Bob" and "Fighter" will become "[MOOAC]Bob" and "[MOOAC]Fighter".
To change your name, please read Manual/Player settings.
Some clans may have the only target to win all the "Clan Wars" they can (mainly "official" matches against other clans), while others may simply want to play together with their friends (mainly "training" matches with the other members of the clan or against casual players).
Some clans may want only "pro" (very skilled) players, while others may also allow new, unskilled or high-ping players (the latters are players with a bad connection).
Some clans may be focused around a specific gametype (for example, Elimination or Capture the Flag modes) or a specific mod (for example Excessive Plus or CorkScrew), while (most) others may like playing in more or all modes.
Often clans create their own game servers and password-protect them (private servers), giving the access passwords only to the clan members and to the other clans that they invite for a specific clan war.
If you like OpenArena, why don't you try to join a clan, or create your own clan, asking your friends to join it?
Hint: once you know a clan's tag, you can search for it (or for a part of it) on active servers (finding out where members of that clan are currently playing) using some external server browsers (like OAFF or the Dpmaster web page itself).
If you want to search more information about a certain clan over the Internet, even if you don't know the "full name" of the clan, you can try searching for the alphanumeric part of the tag (anything except special characters) + the keyword "Openarena" on Google (or the search engine you wish): this may bring you the clan's website. For example, if the tag is [MOOAC], you could try a Google search for "MOOAC openarena" or "MOOAC openarena clan" (also without quotations). In most cases you should find their website, or a website linking to or talking about them.
Tip: to quickly change your nickname, adding and removing your clan tag as needed without re-typing the name each time, you may bind two different "set name <nickname>" commands to two different keys (see command console), or place them in two different configuration files (that you would then launch using the "exec <configfile>" command), or use the "favorites" feature of the nice OpenArena Name Changer external tool.
Hint: clan members may like to use VoIP (voice chat) features to talk each other, instead of writing text: take a look to Manual/Voice chat if you are interested. About text messages, instead: remeber that the game also allows to send text messages to your teammates only: you can set a key for that from SETUP -> CONTROLS -> MISC menu (while the chat command is binded to "T" key by default, team chat command has no default binding).
List of active clans in OpenArena [last update 13th April 2015]:
- /N/ (Nine Inch Noobs)
- H4L. (Hacker for Life "software and hardware")
- chS/ (chicken hunterS)
- GoD (Glad or Dead)
- LoL (Legion of Lollers)
- *M*^ (Mega Clan)
- sDF/ (sexy DeFrag)
- *SoS* (Storm of Stars)
- supeR, Clan
- San Vitus Dance (SvD)
- WASP (Excessive Plus)
- 4MorI (defrag clan)
- WWWD (What would w3sp do? ; Defrag clan)
- RBS (xXREBELSXx)
- KCPI2S! (KillingChildPornIn2Seconds!)
- (alPha) Clan
- |oaN| (Open Arena Noobs) Clan
- OpenArena Clans Book (divided into active, inactive and dead) - by *M*^Clan link is down
- A list of OA's clans and players link is down