Here is a Guide to Wars. If you have any further questions, feel free to ask in the replies of that thread.
They are a friendly contest between two clubs where each club scores points based on how much money they make from hitting players from the other club. The other team has to accept the war, and there are normally a bunch of rules and a well-defined list of people who will be on each side to limit any chances of drama. They can be big events and effective social gatherings - knitting together up to 200 people, and they make for some of the most fun you can have on here! But they are rather difficult to organize and keep drama-free which makes them pretty rare, these days!