While you can use them for anything you would like to, in the majority of cases they are used to store vinegar, vinaigrettes or oil.
The value of a vinegar cruet depends on what type it is and how old it is. The value for older cruets in excellent condition would be more valuable.
They're called "Cruets".
Have a pair of oil and vinegar cruets and not sure how to make use of them? Not to worry - just about any recipe book or website with a section for salad dressings or bread dipping will feature several variations. For the basics, just look for a homemade Italian dressing recipe. Sites like AllRecipes are a good place to start.
Oil and vinegar sets can be constructed by starting with a basic olive oil and a red wine or balsamic vinegar. Dried whole herbs, such as thyme, rosemary and oregano are typically found in upscale oil and vinegar sets found at retailers.
cruets
cruets
I usually use 1 measure of vinegar to 2 measures of oil.
I think it is the category that salt, pepper, mustard, vinegar etc.. fall into. You can get 'cruet sets' that allow all of those substances to be decanted into decorative containers for use on the dining table.
You let it sit and oil will float and vinegar will sink. You then can use an eye dropper .
Oil has a lot, vinegar hardly none. It depends on what oil you use. http://www.thecaloriecounter.com/Foods/400/4034/Food.aspx
Subway typically uses a blend of soybean oil and red wine vinegar in their sandwiches.