@unknown- attacking is a button you press on a players profile to use your own troops to try to defeat theirs. If u win you earn gold and your target loses a higher percentage of troops than you do. But if you lose, you lose a higher percentage of troops and earn no money.
A clan is a group of players following the same basic morales. When u join a clan, you have access to their clan chat to talk to other members. You can only fight estoc wars, system wars and ebs from your clan.