It is just another name for a sausage. Normally in Britain, a sausage is pork, unless otherwise stated, and mild unless otherwise stated. Of course you find beef and venison ones, and combinations of unexpected things, such as pork and stilton cheese, as well ones with loads of herbs and spices. Any and all of these are bangers, but mainly it will be just pork.

Of course, I'm only thinking about real sausages, made with actual meat, not the cheap things, made with things I don't want to think about.