Nausea and vomiting are not diseases. They are symptoms of another condition.
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.