Correspondence is a body of letters or communications. If you've ever had a pen pal or an email buddy, you’ve written plenty of correspondence.
Besides referring to that stack of love letters you've bound up with ribbon, correspondence has a couple of other meanings, too. When you and your friend hold similar opinions about a particular subject, your thoughts are in correspondence, or "agreement." In mathematics, symmetrical objects are in correspondence — that is, they mirror each other exactly.