What this coin looks like (obverse, reverse, mint mark location, special features, etc.): One major variety is the 1840/18 Braided Hair Large Cent (Penny). This repunched overdate error features a small 1840 date struck over a larger 1840 date. Parts of the underling "18" can be seen within the overlying 1840 date as depicted by the arrows in the example image above.
Coin value chart: Typical coin prices, values and worth in USD based on grade/condition