The borders of France and the states ruled by France grew during the wars of the French Revolution and the Napoleonic Wars. On May 12th, 1804 these conquests received a new name: the Empire, ruled by a hereditary Bonaparte Emperor. The first - and in the end only - emperor was Napoleon, and at times he ruled vast swathes of the European continent: by 1810 it was easier to list the regions he didn't dominate: Portugal, Sicily, Sardinian, Montenegro, and the British, Russian and Ottoman Empires. However, while it's easy to think of the Napoleonic Empire as one monolith, there was considerable variation within the states.
The Make-Up of the Empire
The empire was divided into a three-tier system.
Pays Réunis: this was land governed by the administration in Paris, and included the France of the natural frontiers (i.e. the Alps, the Rhine and the Pyrenees), plus states now subsumed into this government: Holland, Piedmont, Parma, the Papal States, Tuscany, the Illyrian Provinces and a lot more of Italy. Including France, this totaled 130 departments in 1811 - the peak of the empire - with forty-four million people.
Pays Conquis: a set of conquered, although supposedly independent, countries which were ruled by people approved by Napoleon (largely his relatives or military commanders), designed to buffer France from attack. The nature of these states ebbed and flowed with the wars, but included the Confederation of the Rhine, Spain, Naples, the Duchy of Warsaw and parts of Italy. As Napoleon developed his empire, these came under greater control.
Pays Alliés: The third level was fully independent states who were bought, often unwillingly, under Napoleon's control. During the Napoleonic Wars Prussia, Austria and Russia were both enemies and unhappy allies.
The Pays Réunis and Pays Conquis formed the Grand Empire; in 1811, this totaled 80 million people. In addition, Napoleon redrew central Europe, and another empire ceased: the Holy Roman Empire was disbanded on August 6th, 1806, never to return.
Nature of the Empire
The treatment of states in the empire varied depending on how long they remained part of it, and whether they were in the Pays Réunis or Pays Conquis. It's worth pointing out that some historians reject the idea of time as a factor, and focus on regions in which pre-napoleon events inclined them to be more receptive to Napoleon's changes. States in the Pays Réunis before the Napoleonic era were fully departmentalized and saw the benefits of the revolution, with the end of 'feudalism' (such as it existed), plus land redistribution. States in both the Pays Réunis and Pays Conquis received the Napoleonic legal Code, the Concordat, tax demands, and administration based on the French system. Napoleon also created 'dotations'. These were areas of land seized from conquered enemies where the entire revenue was given to Napoleon's subordinates, conceivably forever if the heirs stayed loyal. In practice they were a huge drain on the local economies: the Duchy of Warsaw lost 20% of revenue in dotations.
Variation remained in outlying areas, and in some privileges survived through the era, unaltered by Napoleon. His introduction of his own system was less ideologically driven and more practical, and he would pragmatically accept survivals which the revolutionaries would have cut out. His driving force was to keep control. Nevertheless, we can see the early republics being transformed slowly into more centralized states as Napoleon's reign developed and he envisioned more of a European empire. One factor in this was the success and failure of the men Napoleon had placed in charge of conquered lands - his family and officers - because they varied greatly in their loyalty, sometimes proving more interested in their new land than aiding their patron despite in most cases owing everything to him. Most of Napoleon's clan appointments were poor local leaders, and an exasperated Napoleon sought more control.
Some of Napoleon's appointees were genuinely interested in effecting liberal reforms and being loved by their new states: Beauharnais created a stable, loyal and balanced government in Italy and was very popular. However, Napoleon prevented him from doing more, and often clashed with his other rulers: Murat and Joseph 'failed' with the constitution and Continental System in Naples. Louis in Holland rejected much of his brother's demands and was ousted from power by an angry Napoleon. Spain, under the ineffectual Joseph, couldn't really have gone more wrong.
In public, Napoleon was able to promote his empire by stating laudatory aims. These included safeguarding the revolution against Europe's monarchies and spreading freedom throughout oppressed nations. In practice, Napoleon was driven by other motives, although their competing nature is still debated by historians. It's less likely that Napoleon began his career with a plan to rule Europe in a universal monarchy - a sort of Napoleon dominated empire which covered the whole continent - and more likely he evolved into wanting this as the opportunities of war brought him greater and greater success, feeding his ego and expanding his aims. However, a hunger for glory and a hunger for power - whatever power that may be - seem to have been his over-riding concerns for much of his career.
Napoleon's Demands on Empire
As parts of the empire, the conquered states were expected to assist in furthering Napoleon's aims. The cost of the new warfare, with greater armies, meant more expense than ever before, and Napoleon used the empire to for funds and troops: success funded more attempts at success. Food, equipment, goods, soldiers, and tax were all drained out by Napoleon, much of it in the form of heavy, often annual, tribute payments.
Napoleon had another demand on his empire: thrones and crowns on which to place and reward his family and followers. While this form of patronage left Napoleon in control of the empire by keeping leaders tightly bound to him - although putting close supporters in power didn't always work, such as in Spain and Sweden - it also let him keep his allies happy. Large estates were carved out of the empire both to reward and to encourage the recipients to fight to keep the empire. However, all these appointments were told to think of Napoleon and France first, and their new homes second.
The Briefest of Empires
The empire was created militarily and had to be enforced militarily. It survived the failures of Napoleon's appointments only as long as Napoleon was winning to support it. Once Napoleon failed, it was swiftly able to eject him and many of the puppet leaders, although the administrations often remained intact. Historians have debated whether the empire could have lasted and whether Napoleon's conquests if allowed to last, would have created a unified Europe still dreamt of by many. Some historians have concluded that Napoleon's empire was a form of continental colonialism that could not have lasted. But in the aftermath, as Europe adapted, a lot of the structures Napoleon put in place survived. Of course, historians debate exactly what and how much, but new, modern administrations could be found all over Europe. The empire created, in part, more bureaucratic states, better access to the administration for the bourgeoisie, legal codes, limits on the aristocracy and church, better tax models for the state, religious toleration and secular control in church land and roles.