Errors VS. Varieties
What is the difference between an "error" and a "variety"?

In simple terms, a variety is created before any coins are struck.  The dies themselves
contain a anomaly or abnormality.   A doubled die is
not created as the coins are struck.  A
doubled die is created when the die itself is made.  Small dates, Wide AM's, repunched
mintmarks, and over mintmarks are the result of die changes or variations in the die.  The
Red Book defines a Die Variety as "
any minor alteration in the basic design of a coin".
An error is created as coins are struck.  Errors are usually the result of dies being used too
long.  When a die is used too long, die cracks, dropped letters, machine doubling,
laminations, filled-dies, BIE's, clashed dies and other errors occur.  Other forms of errors, like
blank planchets, off-center strikes, chain-strikes, and broadstrikes are the result of the die
not striking the coin correctly.   Errors are generally worth less than varieties.   The Red Book
defines an error as "
a mismade coin not intended for circulation".      
Take the 1995 doubled die for instance.  It is a die variety because there are thousands that
all contain the same characteristics.  If you look at "off-center" cents, there are also
thousands of these, but they do not have the same characteristics?  

Varieties are intentional and unintentional changes to dies before coins are struck.

Errors are unintentional mistakes or abnormalities made during striking.    

What about the 1922 No D (Plain)?
Many collectors consider the 1922 plain an error and not worth collecting.  Others feel that
Die Pair 2 is the only true variety because the D was removed form the die before striking,
while the other three die pairs are errors because they are caused by mint grease.  There is
no right or wrong answer. The question is, do you consider the coin worth collecting?