What this coin looks like (obverse, reverse, mint mark location, special features, etc.):
One major variety is the 1820 Small Date Coronet Head Large Cent (Penny). This coin features a date with smaller numbers. The most notable feature is that the "2" will have a curled tip which makes it easily distinguishable from the large date variety. The difference between the 1820 Small Date vs Large Date penny can be seen in the example comparison image of the coins below:
Coin valuation chart: Typical coin prices, values and worth per grade or condition - in USD