What this coin looks like (obverse, reverse, mint mark location, special features, etc.):
One major variety is the 1851/81 Braided Hair Large Cent (Penny) Overdate Error. This coin features an 1851 date struck over an inverted upside-down 1851 date. This resulted in the "51" being struck over an "81". One way to identify this coin is to notice a small portion of the underlying "8" on both sides of the "5" and parts of the underlying "1" around the upper portion of the overlying "1" as depicted by the arrows in the example image of the coin.
Coin valuation chart: Typical coin prices, values and worth per grade or condition - in USD