Intelligent Design is the argument that certain features of our world are best explained by appealing to intelligence, even though the identity of the designing intelligence remains open. (Contrary to the claims of many atheists, David Hume was an advocate of Intelligent Design.) The Design Argument is the argument that certain features of our world are best explained by appealing to God as designer. Special Creation is the claim that God created the world in a particular way (usually in some way that lines up with the opening chapters of the Book of Genesis).
In this video, William Dembski defines Intelligent Design and contrasts his position with the Design Argument and Special Creation.