Here is a quotation I found on an Intelligent Design website:
"The theory of intelligent design (ID) holds that certain features of the universe and of living things are best explained by an intelligent cause rather than an undirected process such as natural selection. ID is thus a scientific disagreement with the core claim of evolutionary theory that the apparent design of living systems is an illusion."
Basically, it is a way for Christians to try and reconcile evolution and religion. The idea is that evolution is accepted as a theory with the understanding that it is guided by a divine hand. That there is an intelligence behind the design. There is absolutely nothing wrong with this theory. HOWEVER, it has become politicized and some people see it as a way to get religion into the science classroom.