I'm not sure if the question is asking about modern day Greece or ancient Greece.  Obviously the geography between the two time periods won't be different, but geographic difficulties that ancient Greeks suffered are not as drastic with modern day tools.  The question is categorized in the history section of Enotes, so I am going to assume an ancient Greek time period is preferred.  

Greece is a peninsula.  The huge advantage to that is access to water.  In addition to simply being a peninsula, Greece's coastline has many accessible harbor locations.  Having good harbors and water access is good for trade, and trade brings economic stability.  Additionally, the water access provides a stable fishing ground to provide food to people.  

Greece is also quite mountainous.  That makes it a difficult country for attackers to invade.  It's not easy terrain.  However, that mountainous terrain is also a disadvantage.  It is tough terrain for attackers, and it's also tough terrain for the Greeks.  The mountains make trade within Greece's borders difficult.  The mountains also make communication from city to city slow.  Lastly, because so much of Greece is rocky, mountainous terrain, there isn't a lot of fertile soil.  This forces Greece to be very dependent on trade.