Fair Use is a very grey area and this is on purpose. Here is a list of four criteria in determining Fair Use.
- The purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes.
- The nature of the copyrighted work
- The amount and substantiality of the portion used in relation to the copyrighted work as a whole
- The effect of the use upon the potential market for or value of the copyrighted work.
The easiest way to ensure you aren't breaking copyright is to simply get permission to use it.
If you are using it to comment, criticize, or parody the work, it falls under fair use.
Not Fair Use:
- Using an image from google for your blog or post
- Using/posting clips of or entire videos/shows/movies to YouTube (unless it is a short clip including your commentary about said clip.)
- Rewriting song lyrics or "sampling" for a non-parody
- Quoting all/most of a news story
Remember that every country has it's own version of Fair Use. The version you should use is the version from the country of the works origin.
Things that do not make something Fair Use:
- Religious or Non-Profit Use
- Out of Print
- The date of the work
- Personal Use