Variable products eg: A computer hard drive available in 50GB, 100GB or 500GB
A grouped product eg: A computer hard drive set that includes three simple products, the 100GB hard drive, a rugged case and an extra long USB cable.
Unless I am mistaken.
The definition of a grouped product at the top of this page on the WooCommerce documentation is actually very misleading - http://docs.woothemes.com/document/managing-products/