When it's labeled OU-pareve, it's free of milk or meat. When it's labeled
OU, it's also pareve. (See http://www.ou.org/kosher/policy.htm)

A food can be labeled OU-D  because it has a dairy or dairy-derived
ingredient in it, or because it has been manufactured on equipment that has
been used for manufacturing foods containing dairy or dairy-derived
ingredients. Or the food producer simply wants the flexibility to do so in
the future.

Many products labeled OU-D are ok for our dairy-allergic son. But he does
take a risk eating them. Virtually no risk with pareve foods (at least for
milk).