Clan is a meeting place where you can fight together, or challenge another clan group.
While you on a clan, other people can find you, and see when you online. Now days clan are very popular, almost everyone have a clan they fight for.
Some people are loyal to their clan and win clan war to it, some are not and just play to the points fall down.
Those people who have a clan have 3 big letters front of their name, Ex. [EXA]Exampleguy45
There are 3 Ranks in clans. They are: Leader, Moderator, Member
Leader - The one who created the clan and can choose to give other members a higher rank or lower them. He can also invite and kick people.
Moderator - Can invite people.
Member - No ability, just join Clan wars
Some big professional clans have high requirements, Ex. 300k P (Points) 1.20+ KD(Kill/Death Ratio)