Coins minted by the United States have change dramatically in design, size, denominations, and alloy since the birth of the Republic. Whilst the basic units of dollars, cents, and mills has not changed, the relative value of these units is quite different than in 1794, when the first US coin was minted.
The most significant change, other than appearance, in US coins is that over the centuries, our coins have gone from being items of intrinsic value (gold, silver, and copper mainly) to becoming tokens. This has been the general worldwide trend for coinage. One reason is that there is simply not enough gold and silver to go around for coinage any more.