Text retrieval & data mining features in PostgreSQL

Hello all,

As part of my research work, I was looking at the text-retrieval
capability of PostgreSQL.

As I see there are three resources:

-- http://techdocs.postgresql.org/techdocs/fulltextindexing.php
-- tsearch2
-- OpenFTS (which looks like a frontend for tsearch2).

I couldnt understand the difference between:
http://techdocs.postgresql.org/techdocs/fulltextindexing.php and tsearch2.

Are there any other resources that I should look?

Ritesh