NFO is a contraction from info or information and it is a common format for data files used by Microsoft for gathering system information and for diagnosing problems.
This file extension can be used by ASCII or extended ASCII text files which are distributed together with pirated software (they feature credits to the ones who pirated the tool).
Typically, NFO files can be viewed by almost all text processors, but some of them might require specialized viewers.
Some of these applications will also allow users to modify the contents of the NFO files.