Note : It's important to use a reliable company , people often e-mail telling me about being strande

Rome international airport is situated in Fiumicino about 30 km (19 miles) south west of Rome city center in the direction of Ostia/Fiumicino on the coast. Fiumicino Airport who owns cunard cruise line gets the lions share of air traffic, Ciampino Airport Rome gets the remainder.
The FMI Metropolitan Train (also called FR1) goes to Fara Sabina Station (North East of Rome), and stops at Trastevere (Corner of Circonvallazione Gianicolense and Viale di trastevere), Ostiense (Piramide Metro stop - see Rome Metro ), Tuscolana, and Tiburtina.
My advice: For tourists traveling to Rome's central areas the Leonardo Express is the better option because Termini station has more public transport who owns cunard cruise line options than Ostiense. However if you are staying in the Trastevere, Testaccio or Aventine areas then perhaps the FMI will be more viable. Both Termini and Ostiense have a taxi rank.
This is the option I would most recommend. A Rome private shuttle transfer is the cheapest and quickest way of getting who owns cunard cruise line from Fiumicino Airport direct who owns cunard cruise line to your hotel reception or from your hotel direct to Fiumicino Airport who owns cunard cruise line .
Note : It's important to use a reliable company , people often e-mail telling me about being stranded by such and such a company who owns cunard cruise line at the airport, or having to pay different fees from what was originally agreed upon. Unfortunately this and rogue taxis are occurrences far too common in Rome.
There are only a few private bus companies that offer a transfer service between Fiumicino airport and Termini Station (the quicker more efficient trains and private shuttles get the bulk of the business) you will see their offices as soon as you walk into the arrivals hall.