Boolean Full-Text Searches  - http://viralpatel.net/blogs/full-text-search-using-mysql-full-text-search-capabilities/
It may happen that you want to specify certain keywords in your search criteria. Also you may want to ignore certain keywords. Boolean Full-Text Search can used to perform a full text search for such requirements.
If you notice the above Select query, we have added IN BOOLEAN MODE in against(). This query will fetch all the records which has MySQL keyword but not YourSQL keyword. Notice the + and – that we have specified before the keywords!

In implementing this feature, MySQL uses what is sometimes referred to as implied Boolean logic, in which:
+ stands for AND
- stands for NOT
[no operator] implies OR