An old-fashioned or extremely polite word for "woman" is lady. A waiter who's trying to be charming might say, "Can I get anything else for you ladies?"
There are instances when lady is the right word to use, but woman almost always sounds more modern and appropriate. Public speakers and announcers commonly use the phrase "ladies and gentlemen" to address a group of men and women, and if you're meeting a noblewoman, she may very well expect to be called "Lady Gertrude." Sometimes, ironically, lady is used in a rough, rude way: "Hey lady, out of my way!"