Difference between sauce and gravy

All gravies are sauces but not all sauces are gravies. A gravy is a sauce that is normally fortified with a thickener, corn starch and roux, being the most common. A sauce is anything that you put over food that has the consistency of a liquid. Sauces can be sweet or savory, while gravies are mostly dense and can have meat in them. A typical gravy, Americans make/ eat is turkey gravy for Thanksgiving. While a typical sauce would be chocolate sauce.

Not to mention the non food use, meaning 'extra' or a 'bonus'. As in, "If we sell 100 units, the rest is gravy."

Gravy does sort of imply a bit of luxury, something more than just the basic protein.

The term gravy does indeed cover a multitude of sins. "Italian gravy" as a description of a tomato based sauce is more or less unique to a small part of the Eastern U.S. (like New Jersey!). But a more common understanding over here would be sauce based on the juices from a roasting pan, thickened with flour and combined with stock (possibly wine) and herbs for flavor. Then there's this bizarre subset of Southern recipes for things like "Redeye gravy" which combines pan fried ham drippings with black coffee.

Sauce is a very broad term that could include, for example, a cheese sauce, a fruit sauce, chocolate sauce, Mexican salsa, ketchup and more.

Gravy is a specific kind of sauce. Gravy is generally defined, as nutcakes mentions above, as a sauce made from meat or poultry drippings (the juice that congeals in the bottom of a roasting pan.

I understand your confusion, I remember in India that pretty much any savory dish served 'wet' was described as being in gravy. Indian English is great, some of the usages are quite different from what Americans, and British, are used to!

I think it is just a matter of local usage. Of course the French have the classical sauces. In the United States, Italians use the term 'gravy' to describe a long simmered tomato based red sauce that is used to cook meatballs and sausages, then served with pasta that is dressed with the red sauce, often called Sunday gravy. Some people would think of this as spaghetti sauce. In the U.S., aside from the Italian Americans, most people say gravy when they mean a dish made from meat or poultry drippings agumented with stock or water and thickened with flour or roux. This is used over the meat or poultry main dish and often over mashed potatoes. In the U.S. we use sauce to talk about something that dresses a dish or even something to dip appetizers into. Sometimes it is almost a side dish, such as cranberry sauce.