Beginning around 600 BC, each Greek city-state minted (made) its own kinds of coins. (They got the idea from the Lydians in West Asia). A lot of the Greek coins were silver. You made them by taking a small lump of silver and putting it on an iron mold, and then striking it with a hammer that had another kind of mold in it. That way you could squash a picture into both sides at the same time. The Athenians had a lot of silver from their silver mines at Laurion, so a lot of their coins were silver.
Just like today, some coins were worth more than others. This coin is worth less. It’s an obol (it has a centaur on it). An unskilled worker, like someone who unloaded boats or dug ditches, in Athens, would earn about two obols a day – two silver obols a day was approximately minimum wage in ancient Athens.
Even an obol was too much to spend at the store, so the Athenians also made tiny silver coins and larger bronze coins. Eight chalkoi were worth one obol.
All the designs on these coins changed a little from year to year, so numismatists can tell what year (more or less) a particular coin was made, or struck. The government changed the coins so that if anyone cheated and put less silver in some coins, it would be easy to know which were the bad coins.
Eyewitness: Money (Eyewitness Books) (2000).
Sold!: The Origins of Money and Trade (Buried Worlds) (1994). School Library Journal says, “Grade 6-10-Covering primarily the ancient Mediterranean civilizations, this well-written, beautifully illustrated account describes early forms of money, how the first coins were made, and what they reveal to archaeologists about the people who used them.”
Ancient History from Coins, by C. J. Howgego (1995). Not so easy to read, but a good introduction to what we can learn about history from coins.
Greek Coins, by Ian Carradice (1995). This handbook for beginners shows some of the different types of Greek coins, with a little historical background. About a page per century.