Creationism, generally considered to be bullshit, is a religious explanation of existence, specifically, that everything was directly created by some deity (i.e. through magic) and that there is a purpose to all creation.
In recent decades, there have been attempts to invent scientific rationalizations for these beliefs. Intelligent Design and creation science are two labels that have been applied to creationism in an attempt to create scientific legitimacy. The Discovery Institute is the leader in finding quasi-scientific rationale for creationism as described in its Wedge Document.
The best-known pseudoscientific creationism is Biblical creationism, which comes from Fundamentalist Christian theology. Some Muslim fundamentalists also advocate creationist pseudoscience, like Harun Yahya.
Almost by definition all religious beliefs - including those of the deists - are creationist at some level, and not all creationists fall neatly into categories. There are however two main branches: "Old Earth Creationism and Young Earth Creationism.
It is clear that, in the strictest sense of the term, all theists are creationists of some stripe - as they believe the universe was created. However, for some reason many of them dislike the term.
Old Earth Creationists
Main article: Old Earth Creationism
This sect, also known as OEC, generally believes the geologic evidence of deep time, and may accept some form of theistic evolution or Intelligent Design. They generally believe in Noah's flood, although perhaps in some modified, more local, form.
Young Earth Creationists
Main article: Young Earth Creationism
This sect, also known as YEC, rejects the concept of geologic time in favor of Genesis Literalism. They usually rely on Ussherian dates rather than Radiometric dating or other scientific methods. They completely reject evolutionary biology and modern geology. Also, physical constants, such as the speed of light, are often reinterpreted as "inconstant" to explain phenomena such as distant starlight.
Main article: Intelligent Design
Intelligent Design (ID) is an idea fundamental to the "mainstreaming" of creationism. It is the religious argument or belief that complexity requires design.