What is the primary difference between bulimia nervosa from binge eating?

No purging. Binge eating disorder is like bulimia nervosa without the "compensatory" behaviors. People with bulimia always engage in some time of behavior to counteract the purge (e.g. Self induced vomiting, laxative abuse, excessive exercise).
Spectrum. Binge eating can bemiimal to seriusly impairing. Anorexia alwaysis pathological with body image problems as well as contol issues. So binging to anorexia can be seen as on a contuum of disturbance.