A dissenting opinion is written when a justice disagrees with
the majority opinion (which carries the force of law). If a justice
is writing a dissenting opinion, that means he or she voted with
the minority group, and wants to explain the reason why he or she
disagrees with the official Opinion of the Court.
Dissenting opinions may be cited, but are not enforceable.
A good example is if you have 3 people. One of them wants a blue
car, the other 2 want a red one. The majority is the 2 people who
want a red car. Whoever doesn't want a red car, is the dissenting.
(Dissenting is whatever isn't the majority)
Search Dissenting Opinion for more details.