The difference between efficient and effective is that efficiency refers to how well you do something, whereas effectiveness refers to how useful it is.
For example, if a company is not doing well and they decide to train their workforce on a new technology. The training goes really well - they train all their employees in record time and tests show they have absorbed the training well. But overall productivity doesn't improve. In this case the company's strategy was efficient but not effective.
In other words, effectiveness is about doing the right things — things that yield positive results — and efficiency is simply about doing things right i.e. completing a task cheaper or faster.
In the following video, William McDonough, co-author of The Upcycle: Beyond Sustainability--Designing for Abundance speaks about efficiency vs. effectiveness: