From Civilization V Wiki
The game difficulty determines a number of things, including the starting power of your AI opponents, the speed at which your civilization grows, and so forth.
The Difficulty Levels are, in order from easiest to hardest:
Effects of Difficulty
"Prince" is the middle level. On that level neither you nor your AI opponents get any particular bonuses. On levels below Prince you get bonuses in happiness and maintenance costs, and you get better results from Ancient Ruins. Barbarians are less aggressive and less smart on lower levels, as well.