During the 1940’s, the U.S. was facing WWII. Since copper and other precious metals were needed for the war effort, our currency was slightly altered.

The Steel Penny - minted during the war using steel instead of copper,
because copper was needed for artillery shell casings:
http://en.wikipedia.org/wiki/1943_steel_cent

The Wheat Penny -
Very similar to our present day penny, but had a different design on the back:
http://en.wikipedia.org/wiki/Wheat_penny
http://www.usmint.gov/kids/coinNews/coinOfTheMonth/2001/02.cfm

The Buffalo Nickel -
Last minted in 1937, but was widely circulated until the late 1950’s:
http://www.usmint.gov/kids/coinNews/coinOfTheMonth/2001/06.cfm
http://en.wikipedia.org/wiki/Buffalo_nickel

The Mercury or Winged Liberty Head Dime:
http://en.wikipedia.org/wiki/Mercury_dime#Winged_Liberty_Head_.28Mercury.29_.281916.E2.80.931945.29

The Washington Quarter:
http://en.wikipedia.org/wiki/Quarter_(United_States_coin)

Walking Liberty Half Dollar:
http://en.wikipedia.org/wiki/Walking_Liberty_Half_Dollar

The Peace Dollar Coin:
http://en.wikipedia.org/wiki/Peace_Dollar

The Silver Certificate (one dollar note):
http://en.wikipedia.org/wiki/Silver_certificates

Make free use of Google and you’ll find all kinds of trivia about coins!

Good luck in your studies,
~ Mitch ~