Answer:There are two important features to Greek geography that affected the formation of Ancient Greek culture (and thus the Greek people):
Greece is a very mountain region, with only small pockets of land suitable for city-building and the associated city-supporting agriculture. This meant that cities that emerged in the 7th and 8th century BC grew up independently of their neighbors. Thus, unlike the great civilizations of the Middle East (Persia, Babylon), Greek civilization in the classical period grew up as independent "polis" city-states. Although culturally similar (and we refer to them all as "Greeks" as a consequence), Ancient Greeks of this classical period would have identified themselves as Athenians or Spartans, not primarily as Greeks.
A secondary consideration of the mountainous geography was that there was limited agricultural land for the growing of crops. As the cities grew, the leaders of the Greek states began to concentrate what agricultural land that they had upon "cash crops" like olives for olive oil or grapes for wine. For staples like grain, they encouraged colonies in more suitable areas and took place in large scale trade.
When it came to trade, a second feature of Greece's geography gave them an advantage. Much of the Greek region is close to the sea. The Mediterranean (and the sub-seas such as the Aegean) gave the Greeks, especially the Athenians, easy access to the most effective and cheap form of transport, by sea. They conducted lots of trade - getting rich in the process - and became good enough at maritime pursuits to be able to use the navy to fend of a major attempt by the Persians to invade them.
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