Guild system in summoners war has been around since the game has started. Guilds in general is a group of people that has the same interest, goal, etc. They join together to have fun, set goals and help each other, or just want to hang out with other people. In the long run, guilds is a way to build trust within the group. Guilds have someone of higher position than the rest of the group, it's commonly called the Guild leader or Guild Master. He's usually the creator of the guild. 

Recently in Summoners war, a new feature has been revealed to the players. It's called Guild Wars/Battles. In guild battles, every guild will fight other guilds to win and earn guild points and raise their guild ranks to gain better rewards every week. Guild points are used to purchase exclusive monsters in the game, as well as summoning materials and evolution material. You can read Guild Battle Rules HERE.

Some guilds will have their own sets of rules within their guild. This is commonly used to make the guild more organised.