Clans are groups of people who share a common interest in a game. They play together in a friendly atmosphere and sometimes even compete against other clans.
One example is my clan (shameless plug) Warrior Nation, we are one of the larger clans, but still friendly.
Wikipedia is more thorough in it's description:
If you wanna look into it:
Also if you wanna talk more about this or joining, add me to MSN or email me with this address firstname.lastname@example.org