Hahn  is in Rhineland-Palatinate, Germany, about 100 km west of Frankfurt. The town of Hahn is somewhat overshadowed by its airport, Frankfurt-Hahn, used by many budget flights as an entry point to Frankfurt.
Hahn (IATA: HHN)  airfield is a major destination for various budget airlines, mostly notably Ryanair. Recently, the airport officially got the right to call itself "Frankfurt-Hahn", even though it is located about 100km away from Frankfurt (travel time is about 90 minutes by bus).
The place is vigorously being developed and the airport has a nice, modern terminal, full of shops and bars. There are a couple of hotels nearby and more being built. Several major car rental companies have outlets here.
The main contributor to such a change is an Irish budget airline Ryanair, which made Hahn one of its European bases and offers flights to numerous European destinations such as Bologna in Italy and Riga in Latvia.
A number of bus companies operate direct services to Hahn from cities in the region, including Frankfurt, Trier, Luxembourg, Metz, etc. The operators, routes and timings change constantly, so see the official site for up-to-date listings.
Be aware that the bus from Frankfurt airport is often full (as it goes from Frankfurt city and makes a stop at Frankfurt Airport Terminal 1) especially when time is close to popular Ryanair flights to Italy. So, allow plenty of time if having to go from Frankfurt airport to Hahn airport for a connecting flight. If you have big bags, be aware that baggage compartment of a bus can be full, too.
Hahn is located along the B-327, a short distance from the B-50/E-42 between Bernkastel-Kues and Kirchberg.