Every human requires it.
Only Christianity provides it.
With the Aurora, Colorado shootings, the understanding of forgiveness is never more important than now.
Start by reading the article on “Forgiveness” from the International Standard Bible Encyclopedia (see especially section 4). See also Chris Brauns’s book Unpacking Forgiveness, my review (here), a synthesis between Brauns’s work and N. T. Wright’s (here), and my two posts on Miraslov Volf’s work Free of Charge (here and here).