Utilitarianism

The Stanford University's Encyclopedia of Philosophy is a brilliant resource for all the ethical theories. 
It was started by a scholar Ed Zalta whom I was lucky enough to meet at a conference at Stanford University a few years back. 

Far better than wikipedia and, also, I consider this a "quality resource" when it comes to your assignments so you are welcome to cite from here. 
This is a very good and comprehensive overview of Utilitarianism (and consequentialism from a broader perspective).  Reading Section 1 and 2 will give you a very good base knowledge of Utilitarianism.