The largest muscle groups of the body that many people engage during resistance training (lifting weights) are the Quadriceps, Hamstrings, Gluteus Maximus, Pectorials (Chest Muscles), Lats (Back Muscles), Abdominals, Biceps and Triceps.  While working one group of muscle, those muscles are called the primary muscle group.  Additionally, while training those muscles, a second group of muscles called the "assisting" muscle group support the primary muscles which allows for one to carry the motion all the way through.  For example, while one is engaging in a leg extension (when a person is sitting in a machine, lifting their legs upward) they are concentrating their exercise in their Quads.  However, in order for their body to actively engage their quard, their hamstrings are their assisting muscle group that helps to allow their quads to engage in the motion.  Below is a picture of a leg extension.  While this exerciser is engaging their quads, they are also working their hamstrings.