Apart from the three metro lines and the numerous bus lines Rome also has a – rather limited, especially from a tourist point of view – tram system. The main hub is right by the Porta Maggiore city gate. Most of these tram lines take you towards the suburbs and are not very useful for tourists.
Most of these lines do not go towards the center of town, but connect some of Rome’s suburbs with the center. The central tram station then, is not at Roma Termini, but at the Porta Maggiore, a 10-15 minute walk from the main railway station. To get there you just follow the Via Giolitti until you reach the station, which is also rather interesting since Porta Maggiore is also the spot where ruins of many of the ancient aqueducts can be seen.
The most useful tram lines are the number 3 (from near San Giovanni tot Trastevere) and the 19 (from Porta Maggiore to Piazza Risorgimento, near the Vatican City). Especially the latter may be slower than the metro, but more pleasant.
The Rome public transportation tickets can also be used on the trams.
At the moment the tram system is being extended and soon there will be a tram line through a main street, the Via Nazionale. (Update: This moment is now more than 10 years ago, and nothing has happened yet.)
At night, the trams are conducted to their depot, at the beginning of the Via Casilina, just beyond Porta Maggiore.