The NOT operator provides you with a way to exclude unwanted documents from your search. It allows you to search for one keyword or phrase while excluding another keyword or phrase sometimes associated with the first.
If you notice that your query results include many unrelated items stemming from another meaning of your keyword, try using NOT. If you are interested in finding information on flying bats and not baseball bats, try bat NOT baseball.

When to use NOT and when not to use NOT!
NOT is appropriate when a word is used in different contexts. For example, the search vikings not minnesota retrieves records about Vikings but not the Minnesota Vikings. This is what you want if you are studying Medieval Viking sagas. If you are researching pro football, it's not.
The Boolean Operator NOT was used by George Boole to implement Boolean Algebra.