An extensive network of bus lines that covers all of Lazio (the surrounding geographical region of which Rome is the capital) is operated by COTRAL (Consorzio Trasporti Lazio). There are several main bus stations. Long-distance and suburban COTRAL bus routes terminate either near Tiburtina station or at outlying Metro stops, such as Rebibbia and Ponte Mammolo (Linea B) and Anagnina (Linea A).
ATAC, Rome’s city transport service, offers reasonable fares for travel in and around Rome, especially with the BIRG (Biglietto Integrale Regionale Giornaliero), which allows you to travel on all the lines (and some railroad lines) up to midnight on the day of the ticket's first validation. The cost of a BIRG depends upon the distance to your destination and how many "zones" you travel through. Because of the extent and complexity of the system, it's a good idea to consult with your hotel concierge, review ATAC’s website, or to telephone COTRAL's central office when planning a trip. COTRAL has several buses that leave daily from Rome's Ponte Mammolo (Linea B) Metro station for the town of Tivoli, where Hadrian's Villa and Villa D'Este are located. Flixbuses leave from Rome's Tiburtina Metro and train station (Linea B) and will take you to Siena and other towns in Tuscany.
While the bus may be an affordable way of moving around, keep in mind that buses can be crowded due to commuter traffic. Just because you've managed to purchase a ticket doesn't mean you're guaranteed a seat. Make sure to arrive early and stand your ground in line. If you are not able to procure a seat, you may be standing for the entire ride.
If you're taking a city bus, make sure the bus you're waiting for actually runs during that part of the day or on that particular day of the week. For example, notturno buses (late-night buses)—distinguished by the "N" sign just above the bus number—don't run until after midnight and then only a few times per hour. Tourists often get confused while waiting at the bus stop, since the notturno bus schedules are listed side-by-side with the regular day bus schedules. Also, be aware that deviata buses are those that have been rerouted due to road construction or public demonstrations. And festivi buses are ones that only run on Sunday and holidays. Neither notturno buses nor festivi buses run as often as other buses do on weekdays and Saturday. Regular buses will either say feriali, which means "daily," or won't have any special distinction.