I need to know what's an eating disorder in simple terms?

No single disorder: Eating disorders come in several forms that have in common unuusal, potentially harmful eating habits. Most common are anorexia nervosa and bulimia, but obesity also can qualify, as can pica. Anorexics restrict calories and/or exercise until they starve. Bulimics consume huge amounts of calories then purge. Obesity can result from binging without purging. Pica means eating nonfood items.