rpm -Uvh & yum install

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http://serverfault.com – I am new to linux and wants to understand the reason for using some commands. I am trying to install PostgreSQL on CentOs by following the instructions here. First we do rpm -ivh pgdg-centos91-9.1-4.noarch.rpm which installs the rpm package into the system. But why does it ask us to do yum install postgresql91-server (which i assume installs postgresql) again if we have used rpm -ivh to install the package? Additionally, what is the reason for adding the lines exclude=postgresql* into /etc/yum.repos.d/CentOS-Base.repo under [base] and [updates]. Why is it that we exclude yum from accessing (HowTos)