A diner could be a person who dines or eats at a restaurant. A diner could also be a cheap restaurant or snack bar. In California, it makes me think of roadside restaurants where you can get a quick meal. They are not always the cleanest places.

A diner is a cheap restaurant with a particular styles - always with a long bar with with short stools. Many of them are shaped like railroad passenger cars and are metallic on the outside, though not always. Here is a site which gives some pictures of the inside and outside of typical diners. http://www.fotosearch.com/photos-images/diner.html

Usually a cheap restaurant, you can find many of them on the road side.

Like a restaurant, ie. a place where you dine.

A diner is a prefabricated restaurant building characteristic of North America.