What is the difference between Odds Ratio and Relative Risk?

From WHO Basic Epidemiology:http://whqlibdoc.who.int/publications/2006/9241547073_eng.pdf "The odds ratio is very similar to the risk ratio, particularly if a disease is rare. For the odds ratio to be a good approximation, the cases and controls must be representative of the general population with respect to exposure. However, because the incidence of disease is unknown, the absolute risk can not be calculated. An odds ratio should be accompanied by the confidence interval observed around the point estimate". A relative risk is much easier to interpret and makes much more sense to the layman - e.g. a relative risk of 7.0 means that the affected group has seven times the risk of a non-affected group. Most people can grasp this concept fairly easily. An odds ratio (the ratio of the relative odds of the disease occurring in Group A compared to it occurring in Group B) is more complex conceptually, but has some statistical advantages over the relative risk - essentially it's more versatile. The general rule though is that if the prevalence of the disease is <10% or so, the relative risk and the odds ratio will be approximately the same. The rarer the disease, the closer the approximation. If it is statistically sound to do so, it's generally safer, easier and less open to misinterpretation if you use the relative risk, especially if dealing with a less prevalent disease outcome. This is the simple answer anyway.