A 'black & tan' (a co-worker memorably called this "Guinness with Training Wheels") is Guinness mixed with another beer, often a lager. More info here (via Bill of Summa Minutiae):
The first known reference to the expression Black & Tan was in reference to a breed of beagles used as hunting dogs in Ireland. The term was also used to refer to a a regiment of British soldiers recruited to serve in Ireland after the First World War. They had a reputation for being quite brutal and have been accused of many atrocities against the Irish in the years 1919-21.
A good pint can distinguished by a number of methods. A smooth, slightly off- white head is one, another is the residue left on the inside of the glass. These, surpise surprise, are known as rings. As long as they are there you know your're okay. A science of rings is developing - the instance that comes to mind is determining a persons nationality by the number of rings (a ring is dependent on a swig of Guinness each swig leaving it's own ring). An Irishman will have in the region of 5-6 rings (we pace ourselves), an Englishman will have 8-10 rings, an American will have 17-20 (they sip) and an Australian won't have any at all as they tend to knock it back in one go! As you near the end of your pint, it is the custom to order another one. It is a well known fact that a bird does not fly on one wing.
In England, post-operative patients used to be given Guinness, as were blood donors. Sadly, this is no longer the case in England. In Ireland, Guinness is still made available to blood donors and stomach and intestinal post-operative patients. Guinness is known to be high in iron content.