Hi,
you should post this to the
Search forum.
Incidentally, it's name is "Hibernate Search"; giving you a full answer would have me writing some pages.
Can I suggest you to read the book "Hibernate Search in Action", or some article about it?
You should use it if you
a) are using or plan to use Hibernate or JPA, or have the option to use it as you're storing your data in a database
b) have some requirements like "search for products"/"search in this website"/"find articles about X","find by person name" : all those cases in which the search criteria is heavily related to natural language text, in which using SQL is very painful and limiting.
Hibernate Search is able to easily solve problems like "I want most relevant hits as first results"; like to build your own "google engine" embedded in your app/website.