Most trains in Europe have two seating classes. The difference between first and second class is often marginal, particularly on short-haul regional trains. In some cases, though, you can can only reserve seats in first class, meaning that with a second-class ticket on a full train you may end up standing. The differences in classes are greater on long-haul express trains (such as Eurostars).
From the Forums: Are train reservations mandatory in Italy?