Mascarpone is a fresh cheese from Italy which is made from cream coagulated with either citric or tartaric acid. Similar to a cream cheese, this rich, creamy cheese can be used in desserts as well as savory dishes. Probably the most popular dish using mascarpone would Tiramisu. This dessert made with layers of lady fingers, espresso, mascarpone and cocoa powder.
Most of the mascarpone you find in grocery store in the U.S. is made here although some imports can be found in specialty stores as well as online. This fresh cheese product does not keep for very long so it is best to purchase it right before use.
If you don't have mascarpone cheese and you need a substitute or alternative use:
Additionally, you can use of these quick mascarpone cheese substitute recipes that actually heat milk or cream with an acidic buttermilk to make a good approximation of the real thing. If you want to really make the cheese, see recipe link below.
Cream, citric or tartaric acid
If you are up for the challenge (not a huge one) and you have a few cheese making supplies at home, try this making Mascarpone Cheese Recipe. Otherwise, see substitutes for mascarpone above.