What? Ah, lager. The best selling style in the world, that somehow manages to be best selling, most under appreciated, and in some ways one of the most complicated beer to make. Lagering is cold fermenting using a bottom fermenting yeast. It can be as dark as a stout, fruity as a very fruity thing, or bright yellow and malty. What lagering does, is that the yeast produces Carbon Dioxide, which gives your beer a natural fizz as well as all sorts of other assorted interesting flavours.
Where? Germany would probably have been the first place to produce the first versions of lager beer. The Czech Republic became famous for lager with their own drier style; Pilsen. The Brits contributed Pale Malt to the development of the modern day lager in the 1800's. Now most countries in the world produce lager, with the most famous being the United States and Belgium (although we would argue most famous doesn't necessarily mean best).
When? The first mention of cold fermented ales (which would have almost certainly been of a lager style) was in the mid to late 1500's. Since large scale refrigeration equipment is required to create cold fermented ales, Lager spread relatively slowly around colder Northern countries, until the 20th century, when much of the world started making lager.
How? An ice cold glass after work, an unusual dark lager on a tasting board in a bar, lager battered fish and chips. A can of lager while watching some sort of sporting event (preferably where people run about kicking a ball).