i dunno that there is any. aside from maybe a geographical thing.

Well, here in Italy they are totally different things. A bar (snack bar) serves mostly espresso, cappuccino and so on, with "cornetti"

at lunch time they also serve sandwiches or small pre-cooked pizza. They also serve cold drinks, with or without alcohol

typically they serve "aperitivo" at about 11.30/12.00 AM and 6.30/7.00 PM

a pub is open in the evening, there are tables, they serve mainly beer, sandwiches or other kind of food (more or less italianized...)