As a biologist, I am going a bit crazy over the misuse of terms in this thread. To clarify: a herbivore eats plants (well actually, most herbivores mostly eat bacteria, but that is another topic). An omnivore eats plants and meat. A carnivore eats meat. Meat includes all animals, so yes if you eat fish, frogs, snakes, birds or mammals, or lobster, shrimp, scallops, and insects, you are eating meat. You cannot be a vegetarian and eat any of those. Really, technically, you can't eat eggs either, and you are pushing it with dairy.
Re humans - we are omnivores - look at our teeth and gut. This means we have flexibility, just like raccoons (two very adaptable and successful species). So we can eat a range from all plants to all animals, depending on what is available and what we like or are used to eating. Most of us are somewhere in the middle.
We need cholesterol, it is a major component of all our cell membranes, very important for brain function (especially growing brains) and is the precursor for many hormones (especially testosterone, estrogen, and progesterone). It is very hard for our liver to make all we need. We also need essential fatty acids, the fact that they are called essential indicates we can't make them, we have to eat them, just like vitamins.
When we choose to shift to more plant consumption, we have to plan our diet based on the fact that we are still in a predator/prey relationship - in fact my favourite Ecology textbook has two chapters on predator/prey relationships, the first chapter is plants and animals, the second is animals and animals. We have to remember that plants that are tasty and harmless do not do well, evolutionarily speaking, they get eaten. So they have lots of defenses. I can't say plants invented chemical warfare, because bacteria did, but they are very good at it. And what is a plant going to invest the most defense in? Its seeds, so there will be a next generation. This is why we have all sorts of traditional cooking methods that deal with toxins in grains and legumes. And of course modern domesticated plants have had most of their defenses bred out.
General reading - first, the China Study, as already mentioned, is garbage - he cherry picked his data. Read Denise Minger's blog or new book on that. Fat Head is worth watching, and the web site gives lots of other good nutrition blogs. Wheat Belly is worth reading.
And really required reading is The Vegetarian Myth. Its author is Lierre Keith, and she has taken a lot of abuse. She comes at the vegetarian/vegan diet from pretty well all approaches.
OK, off my soapbox. Sorry this was so long. Can you tell that I used to teach Biology?
Where am I in all this? I am low-carb - I was pre-pre diabetic, and now I am fine, plus a lot of other health issues are gone. I am particularly careful with grains (don't eat them at all), because I react very badly to them. Same with mature legumes, I am okay with green beans and snap peas. I eat lots of fat and a moderate amount of various meats, and some dairy (not milk). I eat lots of plants - lots of vegetables and a moderate amount of fruit. All those lovely plant anti-oxidants are fat-soluble - so I make sure I have fat in the same meal so I get their benefit.
