Validity

A deductive Argument is valid iff it is impossible for all premises to be true and the conclusion false.

Why separate form from content?

Validity is about form, not content: a valid form can never take you from truth to falsehood.

Valid + true premises = sound (conclusion must be true)
Valid + false premise = unsound (conclusion might still be true)
Invalid = some way exists for the premises to be true and the conclusion false

Counter-example method

To prove an argument invalid, supply another argument with the same logical form that has obviously true premises and an obviously false conclusion.

Affirming the consequent (invalid):
1. If P, then Q.
2. Q.
∴ 3. P.

Counter-example: 
1. If Snoopy is a cat, then Snoopy is an animal.   (true)
2. Snoopy is an animal.                             (true)
∴ 3. Snoopy is a cat.                               (false: he's a dog)

Validity is the deductive standard. The analogous standard for ampliative arguments is Cogency, since ampliative conclusions go beyond the premises.