In every online game there are "clans", and there are "Clans". The differences between them are:
1) "Clans" will have their own well designed website, which will be gramatically correct and hosted on their own webspace, on a proper domain name. "clans" will either have a website hosted on Geocities or Freewebs, or the website will be permanently "Under Construction", and will never go up.
2) "Clans" will (for the most part) have their own server, or at least a co-owned server with another Clan. "clans" will always be "looking for a server", or be in the process of building one. One will never appear, however.
3) "Clans" will have regular training sessions where AT LEAST half of the clan members will be there every week. "clans" will have scheduled training sessions that nobody turns up to, or won't have training at all.
4) "Clans" will ALWAYS turn up to their matches, with a full strength squad that has been practicing as a team and knows how to play the game. "clans" will rarely, if ever, turn up to their scheduled matches. If a "clan" does make a match, there will be members missing or late, and the squad itself will be under strength. The "clan" players will go to great lengths at the end of the match to explain why you won, and who should have been playing but wasn't there etc etc.
5) "Clans" will have a good way of staying in contact, such as a mass emailer, regularly updated forums or an IRC channel. "clans" will schedule all their matches over AIM or MSN messenger, and if they have a messageboard, nobody will visit it.
As you can see, there is a difference between being in a "clan" and a "Clan". One can ruin your enjoment of the game, and the other will boost it so much that you'll wonder why you played on your own in the first place.