For your convenience and enjoyment, I've created a guide to Indiana's year decals, beginning with the first in 1982 and continuing through the new 2011 decal.
From 1982 to 1992, the word "PASS" is repeated twelve times behind the two-digit year; all this means, simply, is "PASSENGER." Different decals (usually with different color schemes) were created for RV plates, handicap plates and other varieties. Unfortunately, those are now so rare that I have no idea what they look like.
There were actually two styles of decals used for 1998; both were black on deep yellow, although the earlier style was laid out more like the 1995-97 versions. Later decals were numbered to match the plate.
As noted in the image, year decals were used in 1999, 2004 and 2009 ONLY on specialty plates, and that wasn't even universal in those years.