The Hanseatic League (also known as the Hansa) was an economic alliance of trading cities and their guilds that established and maintained a trade monopoly along the coast of Northern Europe (wikipedia). Throughout Europa Universalis 3's incarnations, they have been represented in rather different ways.
Divine Wind and Heir to the Throne
HTTT represents the economic power of the Hansa as an actual playable country and a trade league based in Lübeck. Besides Lübeck, they have political control of Hamburg at the start of the Grand Campaign (eradicating independent Hamburg and decimating Mecklenburg in that scenario). The mighty Merchant Republic is well poised to project their power over much of northern Germany.
In Divine Wind, they were given a third province, Bremen.