Coins minted after 1982 consist of 97% zinc, 3% copper, and ones before that were 95% copper, 5% zinc. This was changed because the intrinsic value of the coin started to rise above one cent. In 1943, during the Second World War, they were made of steel (for a short time) plated with zinc to resist rust.
The cent has gone through several designs over its two-hundred year history. Initially, it was much larger than it is today, about the size of the current United States half dollar coin.
See also: United States coinage