Difference between nerve conduction study and emg?

See below. The EMG portion uses a small needle to directly examine muscle function, and the nerve conduction part uses electrical shocks to measure nerve function. Together the study gives a good idea of the nature of the pathology and location, but can also assess the prognosis and character, and gives often guidance towards an eventual diagnosis.
NCS vs EMG. http://www.webmd.com/brain/electromyogram-emg-and-nerve-conduction-studies.