It’s not just whether a difference is statistically significant that we want to know. We also want to know if a difference is important, because not all statistically significant differences are. For example a treatment might lower blood values of a parameter by a statistically significant amount, but not enough to have a clinical effect. The magnitude of the difference is one of the things that will help us decide how important it is. This is what effect size is.
In this paper from the Journal of Graduate Medical Education Gail Sullivan and Richard Feinn explain what effect size is and how to calculate it. You might like to also look at my previous post on statistically significant and clinically significant differences as well.
Sullivan G M & Feinn R (2012) Using Effect Size—or Why the P Value Is Not Enough. Journal of Graduate Medical Education, 4(3), 279–282. http://doi.org/10.4300/JGME-D-12-00156.1 http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3444174/