Deductive reasoning: Reasoning that starts with a given set premises and draws a conclusion.
Deductive reasoning goes from general to specific by starting with a given set premises and draws conclusions from them. The conclusions are only as valid as the premises that are used and it only one false premise can produce a false conclusion. In a valid deductive argument its truth necessarily follows from the starting premises and it is sound if the argument is valid and all of its premises are true other wise it is considered unsound.
A common form of deductive reasoning is called a syllogism It has three parts.
All X has y.
A general characteristic of a category of objects.
A is X.
The object under discussion belongs to that category
A has y.
The conclusion that the object under discussion has that general characteristic.
Deductive reasoning is that it is heavily dependent on the validity of premises that are used. Premises may be facts derived from observation or a totally philosophical assumption.
For example proponents of the Big Bang Cosmology. All observations of the universe are constant with the Earth being near the center so starting with the philosophical assumption that we are totally the result of natural process; as opposed to being created by God; it is a logically valid conclusion that we can not be in a special place in the universe such as the center. So the invented an explanation for the evidence is to eliminate the center by making all locations seem to be at the center. If we were created by God then we could easily be near the center of the universe. This is why Creationists and Evolutionists can look at the same evidence and draw totally different conclusions.
Deductive reasoning is a very useful tool of reasoning however it does have its difficulties. It depends heavily on the accuracy its starting premises. However flawed premises result in flawed conclusions. These difficulties need to be properly understood to properly understand scientific statements including those of both Creationists and Evolutionists.