Why two packages when installing PostgreSQL on Centos 7?

Question

PostgreSQL

Good time of the day.
Understanding the installation of the postgres database. I found a couple of manuals and a question appeared already at the installation step.
In both manuals, generally understandable actions occur. We connect a fresh package from the add. repositories => update yum list => grab and see what's new.
question for connoisseurs, in one manual installation:
yum install postgresql94-server
in another: As I understand it, the postgresql94-server
yum -y install postgresql94-server postgresql94
package will be important to me - after all, I need a server to work with the database. Then the purpose of the postgresql94 package is not clear . Explain his role, please. If the following package is valid: pgagent_9 x

I understand from the name and you can google its functionality, the question above drove me into a stupor. Thanks in advance for the clarifications.
PS In some manuals, the postgresql94-contrib
package is additionally installed. In general, I really want to understand the functionality of each of the indicated packages.

Answer 1

enter
From the description it is clear that postgresql is needed to connect to the database (even if the database is on the same server), and postgresql-server is the database itself.
Contrib includes extension modules for Postgresql-Server