Facts contain elements that you can measure such as count, aggregate, and perform statistical operations on; while dimensions contain elements that provide context to those measurements. Each logical star has one fact and one or more dimensions. You can manage your own custom star or you can manage a prebuilt star by adding dimensions. You do these operations to extend the model to make use of custom data objects or elements that you've added to the warehouse or to create new calculations or joins to address your reporting needs.