Anorexia doesn't involve vomiting (purging)..that would be bulimia. My doctors have told me that anything involving "purging" calories excessively (ie/ vomiting, excersing all the time, constant movement, etc) is a form of bulimia. Anorexia stems from a latin word meaning "without appetite"- basically anorexics starve themselves. They eat close to nothing or nothing at all, and have trouble eating more. They have an intense fear of gaining weight (which many times can be illogical, ie/ i dont want to get fat so i wont eat the crusts of my bread, or if i eat above 800 calories per day I'll gain weight) Many have a distorted body image (they claim they are fat when in reality they are underweight). A main one is that they stop menstruating, but this is harder to pinpoint if you're worried about a friend or someone you dont really want to ask. If they have stopped though, then it is definately a sign of being underweight. Anorexics also tend to start cooking and reading cook books, even if they never have before (but they never eat the food they make).
I just wanted to add to that, I was diagnosed as having anorexia even with purging. Many anorexics do purge or get rid of food after eating. It is different to bulimia as in the food intake isn't classed as a binge as it's usually lower. It's known as anorexia purging type, or bulimic-anorexia. There are many different types of anorexia and bulimia which are well known to the mediacal profession but not talked about widely enough amongst other people.
If you think you may have anorexia, there are plenty of websites and places that will tell you the signs and symptoms, you may find it helpful.
I've dealt w/ anorexia for the past 8 years. I was a gymnast for about 6 years, started when I was about 7 and was rather good at it, I was even going to start training for the Olympics, but to be a gymnast you have to be perfect. I was never fat by any means, but in a sport like that you are so scrutinized it's unbelievable. I'm 19 now and still have issues w/ food. I'm 5'5" & weigh 133 which is fine for my height, but I still view myself as fat. Anorexia I know is something that you never completely get over, I'll be dealing with this for the rest of my life. Everyday I have to make myself eat, I read labels on everything for how many calories I'm eating and it takes a lot to keep me from jumping back into that trap again....