Voltaire famously quipped that the Holy Roman Empire was "neither holy, nor Roman, nor an empire" — and he had a point. The HRE was a loose confederation of hundreds of semi-independent political entities — kingdoms, duchies, counties, prince-bishoprics, free cities and imperial knights — united under a single elected emperor whose actual power over these territories varied enormously depending on his personality, wealth and political skill. At its greatest extent it covered modern Germany, Austria, Switzerland, the Czech Republic, northern Italy and parts of France and Poland.
The defining feature that distinguished the HRE from every other European monarchy was election. The emperor was not hereditary in theory — he was elected by the Prince-Electors, a select group of the most powerful territorial rulers within the empire. In practice the Habsburgs held the imperial title almost continuously from 1438 to 1806, making it effectively hereditary; but the electoral principle meant the emperor always needed to manage the electors' interests, which prevented the kind of centralised absolutism that France achieved under Louis XIV.
Emperor
Elected by Prince-Electors. Title of Kaiser (Caesar). Supreme in theory, constrained in practice by the princes.
Prince-Electors
Originally 7 (later expanded): 3 archbishops + 4 secular princes. Elected the emperor. Most powerful figures in the empire.
Imperial Princes
Kings, Dukes, Landgraves, Margraves, Counts Palatine — hundreds of territorial rulers with varying degrees of sovereignty.
Imperial Knights
Tiny territories holding directly from the emperor — sometimes just a single village. Subject to no intermediary lord.
Free Imperial Cities
Self-governing cities like Hamburg, Frankfurt, Nuremberg — answerable only to the emperor, not to any territorial prince.
Prince-Bishops
Bishops who were simultaneously territorial rulers — combining ecclesiastical and secular authority in a single person.
The three ecclesiastical electors — the Archbishops of Mainz, Cologne and Trier — illustrate the peculiar nature of HRE politics: the most powerful political actors in the empire were simultaneously its most senior Church officials. The Archbishop of Mainz served as Arch-Chancellor of Germany, the most senior official in the empire. The tension between secular and spiritual power that produced such drama in England was, in the HRE, institutionalised into the very structure of the state.
Ended 1806 — Napoleon abolished the HRE, forcing Emperor Francis II to dissolve it rather than see Napoleon crowned its head. Napoleon called it "an old remnant of a nation which no longer exists." One thousand years of this particular experiment in ordered complexity dissolved in a single proclamation.