You can attack other players to raise your Might and steal resources. You won't steal any resource if you don't win the battle. It is recommended that you upgrade your Intelligence Report research so you can know precisely the composition of your enemy military elements.
Raiding can prove to be a really fast way to collect resources compared to gathering since battles are computed instantly and you don't have to wait for your troops to collect. However you need to make sure you are able the defeat the defence of your target. Also remember that your target will most likely try to strike back and that he may involve his guild.
You can make rally attacks with your guild but beware that all of the target guild members will be notified of the attack, and have the possibility to reinforce your target.