Buttermilk is thicker and more tart than regular milk, which is due to the presence of acid caused by lactic acid bacteria. There is no butter in buttermilk, and surprisingly it is not as high in fat as you might think with nearly half the fat and far fewer calories than whole milk or cream.
If the buttermilk was good enough for my pancakes, then it should be good enough for some muffins I was about to make. I simply replaced the regular milk in the recipe with buttermilk. Amazing! The muffins where so moist, and deliciously decadent. Since then, I've been using buttermilk in most of my baked goods and have been replacing any liquid (water, milk, etc) with it. Not only have I found it improves the texture, but it also combats the grayish discoloring of adding blueberries or walnuts.
My other favorites are making buttermilk salad dressing and marinades. The acid in the buttermilk does a great job in marinades to break down the meat and tenderize.