I would say that a pub is basically a more traditional environment. Draft beer (ale as well as lager) is essential. Pubs don't do cocktails. TVs showing sports, pool tables, dart boards, fruit machines, newspapers and board games are all things you may find in a pub, but not in a bar. The availability of peanuts, pork scratchings, and scampi fries are also good indicators. Usually open during the day. Food, if available, is likely to include English classics including fish and chips, pie and mash, and sunday roasts. A bar would usually have more modern decor. Possibly a greater proportion of standing vs seated space. More of a "nightlife" sort of place... may not even be open during the day. People would get a bit more dressed up, and the music may possibly be louder. There may be a cocktail list. Bottled lagers. Ale on tap would probably not be the norm. Meals are not usually available, but if they are, would be likely to have more exotic aspirations.