Here's the condensed version: In late 1988 some reverse dies of 1989 which had a modified reverse were used intentionally or unintentionally? to mint cents. There are 8 known dies for the 1988 and 12 known for the 1988 D however for some reason the 88 D has shown to be more scarce than the 1988 P.
Technically the variety is known as a Mule, it rode an older version coin onto the new variety that was intended.
There is a great article in the Mega
Red Book and in coppercoins.com the biggest noticeable difference is the flared G on initials and the space between FG and the building. Compare yours to a normal 88 D reverse and its apparent.