Modern genetics proves conclusively that all life on earth is related to a single ancestor, and provides a remarkably accurate way to determine how closely certain species or animal populations are related to each other. Creationists, however, reject evolution and common descent, which poses problems for them with regards to the classification of life. Baraminology is the ham-fisted, pseudoscientific result. It stems, of course, from the bible:
Then God said, "Let the earth bring forth living creatures after their kind: cattle and creeping things and beasts of the earth after their kind"; and it was so.
So God created the different kinds of animals, and you'll often hear creationists talking about "kinds" as if it's a definable term. But it's a vague and imprecise distinction, and we really need better definitions than "creeping things" and "beasts". Baraminologists therefore attempt to classify life into kinds, or "baramin", using techniques shamelessly stolen from evolutionary biology. Since God created the "dog" kind, but not individual breeds of dogs (these are man-made by selective breeding, a genuine form of evolution which even the most hardened creationists are forced to admit to), baraminology has to include in its model some genetic mutation and diversification down the generations. Sound familiar? Check out one of their diagrams:
...which bears more than a little resemblance to the evolutionary "tree of life":
Genetic and physiological similarities are not seen as evidence of common ancestry, because there is no evidence available to refute the possibility that the genetic similarities are a result of a similar design being used on different "kinds."
This is the same flawed logic as saying "the theory of gravity is rejected because there is no evidence available to refute the possibility of God pushing objects downwards".
While the baraminologist's model is basically evolution-lite, and concedes a lot of ground to the opposition (including the occurence of macroevolution), it does go some way to explaining how Noah fitted all those animals onto his ark, since he would only have needed two of each kind, not two of each species. This is the main reason for the popularity of the word "kinds" among creationists, and is a good demonstration of how these ignorami place a greater importance on their ridiculous and childish stories than on the real world.
You can read more about the fascinating world of baraminology here.