Wikipedia has a good write-up on Mint Marks:
https://en.wikipedia.org/wiki/Mint_mark
Another good resource for cents is:
The Lincoln Cent Resource
And a good resource for all coinage is:
Wexler's Coins and Die Varieties
One of the most common things that people find when looking for errors is Machine Doubled Die. This is not only with cents, but with all coinage. It is also not a true error. However, some folks still try to sell some on eBay. There are other forms of such worthless doubling, but these sites explain it a whole lot better than I ever could!