How do you remove white rings left by a glass of water on a wood table?

If the finish has actually been damaged you can buy a finish rub that I’ve only seen at Home Depot that will spare you from local sanding and blending: it’s called restore-a-finish, and you match it to your wood. It rubs right in with a rag. Our house is a 100 year-old arts and crafts, and we were going to have to sand and finish for years — but this stuff just rubs on and hides all of the damage. You can never tell it’s been used.