What is 'couverture' chocolate?

Couverture chocolate contains more cocoa butter than ordinary chocolate and so melts and spreads easily. But cocoa butter is composed of fat crystals, and after melting some of these may recrystallise and form grey streaks in the chocolate called bloom which, though harmless, looks unattractive. So couverture chocolate has to be tempered: that is, heated and cooled to precise temperatures to prevent bloom and then spread and worked on a marble slab as it cools to produce a chocolate with a very smooth and shiny texture. All chocolate ideally needs tempering, but for most everyday cooking it is satisfactory to use good plain chocolate.