Sausages - most often and originally pork - are a traditional way to ensure the slaughtered animal is all used. The intestine would be washed and prepared for use as the casing and the meat used as a filling would be the 'excess' flesh with some fat to make sure the cooking was even and the meat was not too tough.
Made well, a good sausage is excellent. Traditions in different countries led to the German bratwurst and its' friends, the Spanish chorizo , French saucisson, the Polish versions, etc.
A good sausage is also designed to be able to be (air) dried ready to last into the winter - slaughter was most often in the autumn - with the meat sometimes having added herbs or spices according to the regional speciality.
Since the advent of mass consumption, sausages have suffered and are used as a 'butchers dustbin' in some cases, with little meat (30% or even less) and loads of fillers, like rusk and meal.
Check the contents when you buy, make your own - even better - or get the sausages from a reputable butcher who will tell you whats going on in it or a good farm shop whoi know the proveneance of their stock. Continental delis will also have the various different types too, with many being meant to be eaten as a cured sausage of course.
Nice site for info on bangers at:
http://www.sausagelinks.co.uk