Most arguments include three important parts: the claim, the evidence and the warrant. The claim, or conclusion, is the point that you want to prove. The evidence are the individual items that support the claim.The warrant is any kind of aspect to the argument not explicitly stated but assumed to be true. For example, if someone is arguing that car exhaust is causing global warming, the warrant is that carbon dioxide is present in car exhaust.
In an inductive argument, each piece of evidence only increases the likelihood that the final claim is true. Inductive arguments rely on various methods -- such as statistical syllogisms -- to suggest that something is true. For example: If it is true that 89 percent of males like a sport and Frank is a male, then Frank most likely enjoys a sport. The previous argument can be strengthened or weakened with the addition of more evidence. For example, if Frank was also from a middle class family and statistics showed that most middle class families encourage their children to enjoy sports, then the claim that "Frank most likely enjoys a sport" becomes that much more likely.
Deductive arguments are either true or false, with no variation between. Deductive arguments use a series of related evidence to come to a concrete conclusion. An example of a deductive argument might be, "All fish have gills, a koi is a fish, therefore a koi has gills." It is important to note that the addition or subtraction of evidence in a deductive argument does not make the claim stronger. Either all evidence is true and the claim is true, or an item of evidence is false, thereby making the claim false.
A deductive or inductive argument can still be "valid" even if its conclusion is false. Arguments are about providing a consistent structure by which to evaluate a claim, even if the claim is ultimately false. For example, if biologists were to classify some animals without gills as fish, this would render the conclusion of the koi argument false, but the argument would still be valid.
By contrast, a fallacious argument is one in which the conclusion cannot be derived from the evidence. Say we change the aforementioned "koi" example to instead read "All koi have gills, a carp is not a koi, therefore a carp does not have gills." Logicians call this "denying the antecedent." This means it reverses the logical flow of the argument. Inductive arguments have their own fallacies. In our sports example, let us say that Frank is a member of the "Men Against Sports" club. Even though this makes him a minority among men according to the previous evidence, the exclusion of this fact significantly changes the likelihood of the claim that "Frank most likely enjoys a sport."
There are many different fallacies that can undermine an argument. Avoiding them is the key to constructing a argument that, whether true or false, is sure to inspire lively and useful dialogue.