A pitted coin shows raised numerous, often concentrated areas of small raised dots.
VAMworld.com has put together a list of pitted varieties for your convenience.
Pitting is caused by rusting and/or other environmental effects. It typically occurs late in a die's working life, so pitted coins are usually not found with prooflike surfaces. Pitting also typically occurs on the reverse of a coin. This is because in its resting state, the reverse die is on the bottom while the obverse die "hangs" overhead. Therefore, water or other environmental impurities (wow, how often do you hear water discussed as an impurity?) can more easily collect and rest on a reverse die to cause a pitting effect.
Pitting is often seen in multiple forms for one particular date and mint mark (1921-P, 1885-P, 1897-P, 1904-O, and others). Perhaps environmental conditions specific to the mint and the year of strike contribute to pitting.
[corrosion]: pitting that appears on the [surfaces] of coins.
Light [corrosion] is called 'porosity Typically not listable but does help in deteming which die pair was used
Tiny Raised dots usually on the center devices and some believe this to be a a result of a fresh die
that had started to rust while it was stored.
Moderate [corrosion] is called 'granularity, somewhat listable when the devices start to spread into each other
Note the definition of the Ear has started to mush together, same with the Wing Feathers
Heavy [corrosion] is called 'pitting.' and always listable, however does not always appear in the official description
and usually found in the fields of a coin