There are six main types of trains in the state railway in Italy:

• Eurostar
• Intercity/IntercityPlus
• Espresso
• Dirett
• Interregionale
• Regional

The main difference is the speed, number of stops, level of comfort, distance and price. Most of the trains offer first and second class, with first being more comfortable and more expensive. 

Eurostar (ES) is the fastest and most comfortable train, with very few stops in between destinations from North to South. Seat reservation is automatically issued with the ticket without any extra fee. If you purchase your ticket online you will get email confirmation that also serves as your ticket.

Intercity (IC) and Intercity Plus (ICPlus) trains are also comfortable and fast. ICPlus trains are just as comfortable as ES but cost much less. IC and ICPlus also run from North to South but make a few more stops than ES. Seat reservation (€3 fee) is not required on either train.

Espresso (E) makes about the same amount of stops as the IR trains. Seat reservation is not required on E trains but can be issued for €3 fee.

Diretto (D) covers a medium distance, usually within a region or between two neighbouring regions. Reservation is not available on D trains.

Interregionale (IR) trains connect neighbouring regions with stops in large and medium sized towns and not very expensive fare. Reservation is not available on IR trains.

Regionale (R) trains are the least expensive but only run within the region and make many stops. Reservation is not available on R trains.

If time is of concern, it is better to pay a little more taking the Eurostar than waste hours sitting on a train trying to get from one place to another.