While high-speed trains are usually the best choice for getting around Italy, not every city is served by Trenitalia's AV trains or Italo. Most cities and towns in Italy are served by what are called “regional” trains, which are slower and often older trains.
There are a few different names for regional trains in Italy, including Regionale (abbreviated with an R on train schedules), InteRegionale (IR), Diretto (D), and Espresso (E). Don't be fooled by words that look like “direct” or “express,” either – those don't necessarily mean the train trips are any faster than the Regionale trains.
These trains tend to stop at every single station between two larger cities. Despite the fact that all of these are fairly slow-moving, there is an option known as the Regionale Veloce, or “fast regional train,” which makes fewer stops between two destinations.
Regional trains may not be the fastest option, but they're often the least expensive – and in many places they're the only game in town. It's also important to note that most regional trains only have second class cars, require no reservations, and may or may not be air conditioned.