Union Square is a shopping district located in downtown San Francisco, it gets its name from a one block park between Post, Geary, Powell, and Stockton Streets. It is home to pricey shops, hotels, and restaurants. If you're going to ride a cable car while you're in town, this is the place to do it.
The best time to see union square is Christmas, but any time will do really.
The Xanadu gallery is noteworthy because it was designed by Frank Lloyd Wright, however there are many more galleries in the neighborhood.
The Westin St Francis is a large fancy hotel right on Union Square. Expect to pay $350 and up a night for the privilege. You will know immediately upon arrival that you're staying in one of the most luxurious hotels in the city.
A map showing Union Square's location from SanFranciscoShop.com.
Below are various pictures of the Union Square Area, click to enlarge.