Prussia was a Germanic kingdom and state from the 1200s to the 1900s. Prussia's importance peaked in the 18th century as it became one of the five great European empires (the others were Austria, France, Russia, and the United Kingdom).
The Prussian borders changed often. At the height of its power in the mid-1700s, Prussia successfully expanded in size multiple times, thanks to its highly effective military. During this period, Prussia's greatest rival was Austria, because both Prussia and Austria wanted to control the rest of Germany.
The Prussians didn't win every military conflict in which they engaged, however. Prussia played a large role in the French Revolution and the Napoleonic wars from 1803 to 1815. Prussia lost several territories to France as a result of these conflicts, as well as territory to Holland, Russia, and several small nations, including the Duchy of Warsaw and the Kingdoms of Saxony and of Westphalia. And yet, despite the loss of some land, the Napoleonic Wars allowed Prussia to finally emerge as the dominant power in Germany, rising above its long-time rival, Austria.
Prussia went to war with France again in 1870 (in the aptly named Franco-Prussian War). It was during this war that Prussian Prime Minister Otto von Bismarck unified the German states. After this, Prussia was increasingly consolidated into Germany and started losing its distinctive identity. The unification of Prussian and German lands continued and Prussia was officially abolished in 1947.
Of the great European powers of its day, Prussia is the only country that has disappeared completely from the map. What was once Prussia now lies within the borders of Germany, Poland, Russia, Lithuania, Denmark, Belgium, The Czech Republic, and Switzerland.