An nfo file, is a file packed with a release by a group when they release things on the Internet. The files are designed by ASCII artists and can be read with notepad. They contain release information. For example, the runtime of a movie, how to install if it's software, producer, label, tracklists and so on. It's like an identification file, there is always an nfo file packed with a release unless you rip it yourself or get it from a studio or something. The best way to view this info is using a NFO Viewing program. I use DAMN NFO Viewer which u can get here:
SFV (Simple File Verificator) files are used to ensure if files are: - not corrupt - not missing
For example, with RARs it's important that we got all the parts and that none of the RAR files is corrupt... If it is RAR will tell us (if its new RAR that is). However to know that rar file 020 is corrupt we need to have previous 19 files to test... SFV allows us to test any file at any time without the need to have other files.
Further more for mp3s we might never notice a file missing (afterall, other songs will play allright) but thanks to sfv its easy to spot which file is missing.
A nice little program I use for this is QuickSFV. You can grab it here: