The pub is a great place to experience a non-touristy side of British culture, though it can be very different from an American bar. It is important to understand those differences before venturing into this staple of British life and trying to blend in with the locals.
The pub, or "public house," is characterized by its very relaxed atmosphere. They are more family friendly than an American bar, which is something that can come as a shock to an American visiting a pub. It is not unusual to see a family with children enjoying a meal at a pub. In addition, the United Kingdom has a lower drinking age than the United States, so those 18 and up can legally enjoy a drink at the pub too. Even those aged 16 and up can be served an alcoholic drink, provided they are eating a meal and are accompanied by an adult.
One of the most confusing things for American tourists in a British pub can be knowing where to order. While waiting patiently at a table for service in the States is expected, in Britain it means not getting served! Go up to the bar to order and make sure to have cash ready to pay the bartender then.
Pubs are not known for their specialty cocktails, so expect some strange looks if ordering one. They do have plenty of other options though, including various beers, wines, liquor, and non-alcoholic mixers. When ordering beer at a pub it is important to be specific, as they are likely to have several types of beer available. In general, there will be a range of beers from lager (a light beer that would be most like what Americans are used to), to bitter (a light ale), to ale (a dark beer), to stout (very dark beer). Beer in Britain is served in pints, and often less cold than it is in the States.
Making a polite impression on a bartender is never a bad thing in any country. An easy way to do this at a bar in a British pub is to make eye contact with the bartender and then wait patiently. Bartenders can get quite busy but they will get to everyone in turn. Do not wave money around to get their attention, as that can be considered offensive.
One of the biggest differences between pub culture and American bar culture is the concept of tipping. Tipping does not really exist in the United Kingdom like it does in the States. To show appreciation to a particularly helpful bartender, offer them a drink instead by saying, "And one for yourself."
When at a pub with a group of people, only send one person up to the bar to order, so as not to crowd the bar. It is also common at the pub to order in rounds, so everyone in the group will take a turn buying a drink for everyone else.
Lastly, it never hurts to end with a "Cheers!"