A character's alignment is the measure of how good or evil that character
is.  Alignment varies over:

Very Evil - Evil - Slightly Evil - Neutral - Slightly Good - Good - Very Good

   Alignment normally varies over time, depending on the alignment of the 
creatures you kill.  Once you venture past the Slight/Neutral alignments and
become Good or Evil, it is somewhat difficult to become Neutral again.
There are items that may help your transition in either direction.

   Some articles of equipment are anathema to your specific alignment and
cannot be used by you.  It could be of crucial importance to a player though
if suddenly a piece of equipment becomes unwearable at a bad time.

   Clerics have a spell to see alignment, and some objects bestow that
ability on the wearer.