A style of Lager, originating from the German town of Einbeck. The name does not come from the german word bock, meaning goat or ram, but an abbreviation of eimbockbier, "eimbock" simply referring to Einbeck. Nevertheless some brewers use goats or similar on their packaging.
In Germany a minimum 6.4% ABV is required by law. Bock is not a protected appellation and thus bock is now produced throughout the world.
Helles bock is also known as Maibock for the time of year at which it is traditionally drunk. It is pale and has a strong malt flavour.
Dunkel bock is dark and is also malty, but also with a chocolate, roasted flavour from the malts used.
Good examples of both styles are produced by Einbecker Brauhaus, sold as Ur-Bock (original bock) to distinguish it from bock produced elsewhere.
Even stronger versions of bock exist in doppelbock and eisbock.