In logic, a valid argument is one that is correct in form. It does not need to be correct in content; it is only necessary that the conclusion does follow from the premises, following the rules of logic.
1) Nixon was French.
2) All French people are purple
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3) Therefore, Nixon was purple.
This argument is not sound, because both of the premises are false. It is valid, because the conclusion follows from the premises.
Note that you can have an argument that is valid but not sound; you cannot have an argument that is both invalid and sound. Sound is the supercategory containing the values of 'valid' and 'true'.