Burrata is a fresh chhese, made from (sometimes buffalo) cow milk. It has an outer shell of mozzarella, filled with Stracciatella. Stracciatella is a kind of stretched-curd cheese, made with a hand-shredding technique. Burrata (and stracciatella too) is a product typical of Andria, in Puglia, Southern Italy. It is produced by combining milk, whey and rennet. Burrata may look similar too mozzarella, but is softer, and much more buttery-tasting. You can find it in different sizes.
Burrata needs to be served fresh, within 24 hours. You can serve it with salads, or together with cold cuts and more cheeses. But it can also be used to cook more complex dishes, like pasta, or creamy soups. In that case, you can use it within 48 hours too.