What is a vegetarian (vegan) diet?

In one line

A vegetarian diet leaves out meat and fish and is built on plant foods; a vegan diet is the strictest kind, leaving out every food that comes from animals, including eggs, milk and honey.

What is a vegetarian (vegan) diet?
Are 'vegan' and 'vegetarian' the same thing?
You often see the word 'vegan' on restaurant menus and snack packs. Vegetarian sounds like simply not eating meat, so what makes vegan different? Let's look at the kinds of vegetarian diets and the nutrients to watch, step by step.
In a word
No meat or fish; mostly plant foods
A vegetarian diet leaves out meat such as beef, pork and chicken, and fish and seafood, and is built on plant foods such as vegetables, fruit, grains, beans and nuts. A vegan diet is the strictest kind of vegetarian diet and leaves out every food that comes from animals.
Kinds of vegetarian diets
Split by what is added
Vegetarian diets are split by what is eaten on top of plant foods. Eating dairy such as milk and cheese makes it 'lacto', and eating dairy and eggs makes it 'lacto-ovo' vegetarian. A 'pescatarian' diet, which includes fish and seafood, is close to vegetarian but is not strictly vegetarian because it includes fish.
Vegan
Even eggs, milk and honey are left out
Vegans eat no meat or fish, and also no eggs, milk or dairy, or honey, nothing that is taken from animals. So a vegan diet is made only of plant foods. It leaves out the most foods of all the vegetarian diets.
What is eaten
Protein from beans, tofu and nuts
A vegetarian diet is built from vegetables, fruit, whole grains, beans, nuts and seeds. The protein that meat used to supply comes from soy foods such as tofu and soy milk, beans such as lentils, nuts and whole grains. Getting plenty of protein without animal foods is not hard.
How it differs from eating meat
Less saturated fat, more fiber
Vegetarians usually eat less saturated fat (the fat that turns solid, like butter and meat fat) and fewer total calories than meat eaters. They eat more fiber, potassium and vitamin C instead. That is because they eat a lot of vegetables, fruit, whole grains and beans.
Nutrient to watch ①
Vitamin B12 is almost absent in plants
Vitamin B12 is needed to make red blood cells and to protect nerves. It is found in animal foods and almost not at all in plant foods, so vegans had lower blood B12 levels than meat eaters. So vegans get B12 from 'fortified foods' with B12 added, such as some cereals and soy milk, or from supplements.
Nutrient to watch ②
Iron from plants is absorbed less
Iron is used by the blood to carry oxygen. Taking in food parts through the gut into the body is called 'absorption', and the body absorbs iron from plants such as beans and spinach less well than iron from meat. Eating foods rich in vitamin C, such as oranges and tomatoes, at the same time raises iron absorption.
Other nutrients
The more foods left out, the more to watch
Calcium, zinc, vitamin D and omega-3 are also named as nutrients to watch on a vegetarian diet. A vegetarian diet with dairy and eggs supplies all the nutrients needed. But the more foods are left out, the harder some nutrients are to get.
What research found
Less heart disease, more stroke
In a study in the UK that followed people for many years, vegetarians had about 1/5 less heart disease from blocked heart blood vessels than meat eaters. But they had about 1/5 more strokes, mostly bleeding strokes where a blood vessel in the brain bursts. This was a record of people eating as they chose, not an experiment that set their diet.
Common myth / Is any plant-based diet healthy?
Results split by what was eaten
In a US study that followed people for many years, people who ate the most plant foods such as whole grains, vegetables, fruit and nuts had about 1/4 less heart disease than those who ate the least. People who ate the most refined grains such as white bread, sweet drinks, fries and sweets had about 1/3 more heart disease than those who ate the least.
Summary
No meat or fish; vegans leave out all animal foods
A vegetarian diet leaves out meat and fish and is built on plant foods, and a vegan diet is the strictest kind, leaving out even eggs, milk and honey. In studies that followed people, those eating plant foods mostly as refined grains, sweet drinks and sweets had more heart disease, not less. Related terms: plant protein, dietary fiber, micronutrients (vitamins and minerals).