The fastest trains on the Ferrovie dello Stato Italiane (FS), or Italian State Railways, are the Freccirossa. Their privately owned competitor, Nuovo Trasporto Viaggiatori (NTV), or Italo, also runs high-speed service between all major northern cities and as far as Salerno in the south. Seat reservations are mandatory for these bullet trains, just as they are for the Eurostar and slower Intercity (IC) trains; tickets for the latter are about half the price of those for the faster trains.
Reservations are not available on Regionale and Espresso trains, which are slower, make more stops, and are less expensive. For these trains, you must validate your ticket before boarding by punching it at a wall- or pillar-mounted yellow or green box—if you forget to do this, find a conductor immediately. Fines for attempting to ride a train without a ticket are €100 (€50 if paid on the spot) plus the price of the ticket.
A rail pass can save you money on train travel, but it's good to compare the cost with actual fares to make sure. Generally, the more often you plan to travel long distances on high-speed trains, the more sense a rail pass makes. Keep in mind that even with a rail pass you still need to reserve seats on the trains that require them.