Both mostaccioli and penne are tubular pastas with ends that are cut at an angle. The difference between the two is the surface of each. Mostaccioli (or little mustaches) have a smooth surface. The name mostaccioli is found only in the U.S.; the same pasta in Italy is called penne lisce.
There is, however, a ridged mostaccioli called mostaccioli rigati, which is just…penne. Yes, they are the same. The name penne (or pens, because of its quill shape) is more universally known. The ridges of penne allow the pasta to cling to sauce.