PostgreSQL and MySQL are not all that different, in general. I'm not sure why PG was specifically selected where I work, though I understand there are some subtle licensing differences that may have played a part.
Table joins are an integral part of using a relational database. They allow you to query across multiple tables (even across multiple databases). As a simple(?) example, let's say you have "user" and "post" tables:
user_id (int, primary key, auto-increment)
post_id (int, primary key, auto-increment)
user_id (int) -- indicates who did the posting
Now you want to display the latest post, along with the author's name:
SELECT user.first_name, user.last_name, post.title, post.post_text
INNER JOIN user USING(user_id)
ORDER BY post.post_timestamp DESC
If that does not make sense, then it's time to start googling for tutorials and such on relational database design and normalization.