Rice wine vinegar (rice vinegar) is a mild flavored vinegar made from rice with and has less acidity than other types of vinegar. This product is used extensively in Asian cooking. It is a good mild vinegar to use in salad dressings. Available in any Asian grocery store and many well-stocked grocery stores.
Although sometimes referred to as "rice wine vinegar" this vinegar is not made from rice wine but simply from rice. Depending on your grocery store you can look for it in the Asian Food section of the store. You my find the ingredient labeled either way; rice vinegar or rice wine vinegar.
Don't confuse rice vinegar with sushi rice vinegar which is a highly sweetened product used for making sushi rice.