Jazz great Louis Armstrong apparently sometimes closed his letters with the salutation "Red beans and ricely yours", but that's not the only reason to eat red kidney beans. Like all beans, the various colours and sizes of kidney beans are rich in nutrients but low in calories, high in fibre and a good source of protein, vitamins, and minerals.
Red kidney beans are the variety of kidney bean I'm most familiar with. You may have had them in chili con carne or in the Cajun staple red beans and rice, and they're not bad as refried beans in a pinch. Red kidney beans are firm and medium-sized; they have a dark red skin, cream-coloured flesh, and a full flavour. They are also, as epicurious.com puts it, "an enthusiastic producer of flatulence", so consider yourself warned. (Discarding the soaking water from dried beans, or soaking canned beans for an hour and then discarding the soaking water, will help reduce the flatulence factor.) You can substitute any red bean for red kidney beans. I have never seen them fresh, and don't know where they grow; I've only ever encountered them dried or canned. To cook them from dried, soak them overnight, then simmer them for 1 to 1-1/2 hours.
White kidney beans, also called cannellini beans, are milder in flavour than the reds; they are often a feature of minestrone soup and bean salads. They have a nice smooth texture. Use white kidney beans in any recipe that calls for white beans; they're similar to Great Northern beans, which make a good substitute. You might find them fresh, but more usually they're dried or canned; to cook from dried, soak overnight and then simmer for about 45 minutes.
Flageolets are the fancy form of this hard-working bean crowd; favoured by the French, flageolets are dried immature kidney beans. They're small, light-coloured ovals that are similar to white kidney beans but more chic. They are classically paired with lamb, and prepared simply to showcase their delicate flavour and texture. You might find them fresh if you have a really fancy grocer, but usually they're dried, canned, or on occasion frozen. Cook as for white kidney beans.