Intelligent design, more specifically, is the reformulated conjecture that various features of our world are best explained by the existence of a so-called intelligent creator.  The central meat of intelligent design states that the scientific theory of evolution is surpassed by the fact that organisms are composed of biological systems that are irreducibly complex.  They explain that certain components of organisms confirm the existence of God because they are so complex, and all of their component parts are so focused on a particular job, that it would have been impossible for them to have evolved to become so complex.  Just like a clock requires a clockmaker, explain proponents of intelligent design, organisms have certain biological functions which needed a clockmaker.

Advocates of intelligent design have several examples which they claim are irreducibly complex.  The system by which vertebrates coagulate blood, for example.  The existence of eyes.  And the intelligent design proponent's favorite: the flagella of tiny bacterial organisms which have complex organic motors that require the interactions of dozens of proteins.

In peer-reviewed science, all intelligent design claims for irreducible complexity have been soundly shot down.  There are no examples that scientists have not been able to explain through possible routes of evolution.