Formal and informal logic

Formal logic abstracts the form of an argument from an instance of it that may be encountered, and then evaluates the form as being valid or invalid. The form is the important thing, rather than the concrete instance of the form. Informal logic, on the other hand, evaluates how an argument is used in a given context of conversation. This more practical, real-world orientation requires more judgment in interpreting what a given text of discourse should be taken to argue.