While the Mediterranean is home to many different kinds of food culture, when people talk about 'the Mediterranean diet' they tend to have a particular type in mind. Generally, the term refers to a diet using a lot of fruit and vegetables and cereal, with fresh fish and less meat, and with use of olive oil in cooking. Dairy products are also used but not in large amounts, and typically wine is considered an integral part of a meal, but again not drunk in large amounts.
Foods like these are especially typical of Italian (especially southern), Spanish and Greek cookery, as well as some areas of France such as Provence. Doctors and nutritionists generally agree that this kind of diet is one of the healthiest in the world. It is also very tasty; these are all countries and regions that are serious about cooking.
If you increase your fruit and vegetable intake, eat more pulses and cereals, and reduce your meat and dairy you are already on the way to a Mediterranean diet. You can find some recipes here.