Nausea is the very unpleasant feeling of being about to vomit. Vomiting is the spitting up of the contents of the stomach. It's associated with a feeling of nausea and strong contractions of the abdominal muscles. Vomiting is different from regurgitation, in which one spits up stomach contents without feeling sick and without strong muscle contractions.
Vomiting -- especially if it comes with diarrhea -- can cause dangerous dehydration. This is more likely to happen to children. When caring for a sick child, be alert to symptoms that mean the child needs water: dry lips and mouth, sunken eyes, rapid pulse, rapid breathing, and, in babies, a sunken look to the soft spot on the top of the head an no wet diapers.
Sometimes the cause of nausea and vomiting is psychological. This doesn't mean the symptoms aren't real. A qualified clinical psychologist or psychiatrist can help. People who often make themselves vomit may have an eating disorder. This can be a life-threatening condition. If you eat a lot and then make yourself vomit, you need professional help.