Describe the features of bulimia nervosa.?

Binge and purge. Features include binging (eating more food than normal); followed by purging (self induced vomiting).
Binging and Purging. Bulimia nervosa is defined as bingeing (eating more than a typical person would in a similar situation), then using a compensatory behavior (ie vomiting, excessive exercise, laxative abuse) in an attempt to prevent weight gain from the binge. Bulimics can be any size, though typically are normal or slightly overweight. Either way, self image is unduly influenced by weight.
An add on as to WHY. Body image has been an integral sense of identity and value for many thousands of years. Women mostly are more valued in our culture for being the "norm" in the wt category. They tend to be regarded as more valuable financially in their job etc. Women who approach the norm -avg weight regardless of their ablities receive better pay than similar male counterparts.