Here is a useful article - part of a series by Australian teachers - about the types of trains and buses in Italy:
Types of trains (tipi di treni):
Eurocity (EC) and EuroNight (EN) - intercontinental connection with Switzerland, Austria, France, Spain, Croatia, Slovenia, Prague and Budapest.
Eurostar (ES) and Intercity (IC) - fast train between Italy's major cities. You need to pay a supplemento rapido - an extra charge - when you travel on these trains and prenotare un posto - reserve a seat - on Eurostar (ES*) trains. Eurostar trains are all high speed trains, very comfortable but more expensive.
Rapido (R) - a long distance express train but be careful of the Espresso trains!
Espresso (E) - stops at all the major stations, so it is slower than the Rapido!
Inter-Regionale (IR) - travels between regions and sometimes is in service only on weekends and holiday periods.
Regionale Diretto (D)- travels at faster speeds than the Locale and stops at fewer stations; this train travels within a region and bordering regions e.g. In Tuscany or between Tuscany and Umbria; and indicates that there is no need to change trains to reach the final destination.
Regionale Locale (L) - slow train which stops at every station within a region e.g. from Florence to Siena, it stops at all stations in between, even the tiniest towns.
There are also some useful phrases for traveling by train and links to audio files so you can hear the proper pronunciation.