Deductive reasoning: conclusion guaranteed
Deductive reasoning starts with the assertion of a general rule and proceeds from there to a guaranteed specific conclusion. Deductive reasoning moves from the general rule to the specific application: In deductive reasoning, if the original assertions are true, then the conclusion must also be true. For example, math is deductive:

If x = 4
And if y = 1
Then 2x + y = 9

In this example, it is a logical necessity that 2x + y equals 9; 2x + y must equal 9. As a matter of fact, formal, symbolic logic uses a language that looks rather like the math equation above, complete with its own operators and syntax. But a deductive syllogism (think of it as a plain-English version of a math equality) can be expressed in ordinary language:

If entropy (disorder) in a system will increase unless energy is expended,
And if my living room is a system,
Then disorder will increase in my living room unless I clean it.

In the syllogism above, the first two statements, the propositions or premises, lead logically to the third statement, the conclusion. Here is another example:

A medical technology ought to be funded if it has been used successfully to treat patients.
Adult stem cells are being used to treat patients successfully in more than sixty-five new therapies.
Adult stem cell research and technology should be funded.

A conclusion is sound (true) or unsound (false), depending on the truth of the original premises (for any premise may be true or false). At the same time, independent of the truth or falsity of the premises, the deductive inference itself (the process of "connecting the dots" from premise to conclusion) is either valid or invalid. The inferential process can be valid even if the premise is false:

There is no such thing as drought in the West.
California is in the West.
California need never make plans to deal with a drought.

In the example above, though the inferential process itself is valid, the conclusion is false because the premise, There is no such thing as drought in the West, is false. A syllogism yields a false conclusion if either of its propositions is false. A syllogism like this is particularly insidious because it looks so very logical–it is, in fact, logical. But whether in error or malice, if either of the propositions above is wrong, then a policy decision based upon it (California need never make plans to deal with a drought) probably would fail to serve the public interest.

Assuming the propositions are sound, the rather stern logic of deductive reasoning can give you absolutely certain conclusions. However, deductive reasoning cannot really increase human knowledge (it is nonampliative) because the conclusions yielded by deductive reasoning are tautologies - statements that are contained within the premises and virtually self-evident. Therefore, while with deductive reasoning we can make observations and expand implications, we cannot make predictions about future or otherwise non-observed phenomena.