Louvre and match making

So how do you get a foreign work of art for the Louvre. What is the difference between a great work of art and a artifact. I know that weapons and ships are artifacts and assume coins and necklaces are too. So how do I get a foreign work of art?

Go to the culture overview (not the policy overview) and there is a tab that says 'swap great works' or something like that, and there are columns for each GWAM. To swap with someone, click on a cell in your row to put a work/artifact there, then click on another civ's item in that same column and there will be an option on the bottom to swap.

EDIT: There is really no difference except for getting various theming bonuses, and the fact that only artifacts can be discovered by archaelogists.

Another way to get them is to send an archaeologist to foreign lands, but civs might get mad at you.