From the World Heritage inscription:
Prague is one of the most beautiful cities in Europe in terms of its setting on both banks of the Vltava River, its townscape of burger houses and palaces punctuated by towers, and its individual buildings.
The Historic Centre represents a supreme manifestation of Medieval urbanism (the New Town of Emperor Charles IV built as the New Jerusalem). The Prague architectural works of the Gothic Period (14th and 15th centuries), of the High Baroque of the 1st half of the 18th century and of the rising modernism after the year 1900, influenced the development of Central Europe, perhaps even all European architecture. Prague represents one of the most prominent world centres of creative life in the field of urbanism and architecture across generations, human mentality and beliefs.
Prague is one of those great cities where the entire city is lumped together as one World Heritage Site. There could be 3 or 4 separate World Heritage Sites in Prague if they were evaluated on their own merits, similar to Kyoto, Paris or Rome. Expect to spend multiple days in Prague to even get close to seeing the highlights in the city.
View my complete list of UNESCO World Heritage Sites