Gravy is an unctuous sauce, usually meat based and starch thickened. Though the term has gotten pretty loose, you can generally think of gravy as a product of roasting.
Collect the drippings of a roast, deglazing the roasting pan with alcohol. Then, separate the fat from these drippings and use it to make a roux. Stir the skimmed drippings into the roux and season. Any combination of elements may be added along the way to match your style of cooking and the products you're working with.
You can certainly skip the roasting part with just some fat and a little stock. Still, don't overlook gravy's original intent, to make good use of products like fat, drippings and innards, which may otherwise be discarded.
I tend to avoid flour in favor of cornstarch or arrowroot when thickening sauces, but flour responds well to heat, and so becomes essential to a true gravy.
This recipe scales readily, and represents more an illustration of a technique than a hard and fast rule, so feel free to adjust it to use what's on hand. A true gravy should never take you out of your way.