Creationism is a religious belief, usually associated with fundamentalist Christianity, which holds that the universe, life on Earth, and especially humanity, were created by a supernatural being. Biblical Creationists base their belief on the creation story in the book of Genesis, whereas so-called Creation Scientists claim to provide scientific evidence for their belief. Creationism is characterized by its rejection of scientific accounts of the origin of the universe, the formation and development of the Earth, and most particularly, the theory of evolution.
There are several different varieties of creationism, varying in the extent of their Biblical literalism. The most extreme is Young Earth Creationism, which holds that the Earth was created less than 10000 years ago, with living kinds – including man – much as they are today. Geological features are explained in terms of Noah’s flood. Old Earth Creationism allows that the universe is billions of years old, and that life developed gradually, but still rejects evolution, holding that the Creator must intervene regularly to create new forms of life. Intelligent Design Theory is the most recent variant of Old Earth Creationism (though its proponents deny this), allowing a significant role for evolution, while still insisting that supernatural intervention is required.
The evidence against the Young Earth hypothesis is overwhelming, and comes from a wide variety of scientific disciplines. Old Earth Creationism, in all its forms, is chiefly a negative hypothesis stating that the theory of evolution (and science in general) cannot account for the origin and development of living forms. From the (alleged) failure of proposed naturalistic accounts of the origin of species, it is inferred that a supernatural being must be responsible. This argument is fallacious, since the failure of one naturalistic account in no way demonstrates that no naturalistic account is correct.
Moreover, the arguments against evolutionary theory are generally very weak. (There is a huge number of such arguments; for a thorough review and refutation of each one, see Talk-Origins.)
Most creationist arguments against evolution amount to claims that there is not enough evidence for the theory, or that expected evidence is missing. For example, it is often claimed that the fossil record used to justify claims of gradual evolution is full of “gaps”. Creationists demand evidence of “transitional forms” – a species that is half-bird and half-dinosaur, for example. This argument ignores the discoveries of feathered dinosaurs, fish with limbs, and early hominids. Most of the supposed gaps have been filled in with an array of transitional species. More importantly, since fossilization is such a rare phenomenon, the failure to find such fossils would not have constituted evidence against evolution.
Creationism is a religious dogma masquerading as a scientific theory. Its claims largely consist of criticisms of evolutionary theory. Even there, the criticisms are weak, for they mostly involve simply pointing out gaps in the evolutionary evidence, or currently unexplained phenomena. It is a big step from the claim that a piece of evidence or explanation is currently lacking, to the creationists’ claim that it will always be lacking. Given the rate at which scientists have been filling in the evolutionary jigsaw, it is highly likely that the creationists’ gaps will continue to shrink. Meanwhile, outside of scriptural literalism, there is not a jot of positive evidence for the creationist hypothesis.