While both Athens & Sparta were ancient Greek city-states inhabited by people who shared a similar language, religion and a pride in being "Greek," there were distinct differences. The Athenians placed a premium upon having a "sound mind & body," emphasizing a well rounded education to develop a population of analytical thinkers. Economically, the Athenians were a merchant people who had the largest fleet in all of Greece and their city was a cosmopolitan metropolis & center of culture in the ancient world. Athens was a complete democracy, where the entire citizen body voted on all governmental decisions (even though those who qualified as "citizens" actually were a minority of the entire population). As far as military strength goes, Athens was a naval power.
The Spartans established a totalitarian oligarchy, creating a society which taught obedience & discipline. Sparta was an agricultural society which deterred outside contact & influence as much as possible, making outsiders feel unwelcome in an effort to preserve their rigid, structured society from cultural diffusion. Sparta had the most formidable army in all of Greece, despite its limited population.
These two were the strongest city states in all of Greece, until their inevitable conflict during the Peloponnesian War of the 400's BC which saw Sparta overrun Athens. Unfortunately, the victory was short lived, as both city states were weakened by the bloody & destructive conflict, plunging the entire Greek world into chaos and a compromised state. A condition that was soon exploited by Philip of Macedonia, the father of one Alexander the Great.