What is the difference between a restaurant and a diner? If I was standing there looking at it how would I know which one it was?
On a slightly off, but related topic: I've started watching Drive Ins, Diners and Dives on Food Network and I have to say some of those places look amazing!
That show pretty much will show the difference. diners have an all steel & formica & vinyl look. No paying $2 extra a plate for them to wash fancy table cloths. It's a wipe down of the booth's formica table top, lay down some paper placemats & onto the next customer. They look a lot like Mel's Diner from Happy days, even if they don't quite have the retro theme.
They may also still have the old jukeboxes at each booth, so you can listen to your own music at your table.
And in the corner by the kitchen counter is always a Ketchup contraption, in which they place a ketchup bottle in, upside down and it funnels into another bottle so the can can refill the bottles & always have full ones on the tables.
There is also a large glass, lighted cooler filled with round turning shelves filled with several of their specialty pies & cakes.
The foods aren't fancy. They usually have an overabundance of tripel decker club sandwiches to choose from, assorted salad plates, and lots of burgers & toppings, open-faced sandwiches, ice cream fountain specialties and breakfast served 24/7.
Then on the other menu pages there is a list for chops & steaks, Italian dishes, Greek dishes, etc., But, you don't go there because of these. It's not their "specialty" items, like in a restaurant, where you go specifically for the pasta. But those diner dishes may actually be quite good because they've had years to work on a standard item.
I went to a 24-hour one a lot when I was in college.
The 24-hour ones are the best!

The bars & clubs here close at 4 am. So, some people head to a 24 hr coffee shop or diner and have breakfast, get home around 6 - 8 am to go to sleep or get ready for work.
They are also great for first or second dates. When you've been out on your date, like a movie, then actually went & had coffee, desserts or drinks somewhere after. You realise you really hit it off, you both don't want the evening to end, aren't ready to go home yet, and it's too early in the relationship to invite the other person over and give mixed signals. You can sit in a diner, order breakfast and literally sit in the booth and talk all night!

: They don't try to push you out late at night. The waitress just keeps refilling your coffee and you give her a BIG tip for the extra booth time, and everyone is happy.