sudo - execute a command as another user

DESCRIPTION
sudo allows a permitted user to execute a command as the superuser or another user, as specified in the sudoers file. The real and effective uid and gid are set to match those of the target user as specified in the passwd file (the group vector is also initialized when the target user is not root). By default, sudo requires that users authenticate themselves with a password (NOTE: by default this is the user's password, not...

Pls go through it, no more doughts remain regarding yum commands:

To download and install all updates type the following command:
# yum update

List all installed packages, enter:
# rpm -qa

# yum list installed

Find out if httpd package installed or not, enter:
# rpm -qa | grep httpd*
# yum list installed httpd

To check for and update httpd package, enter:
# yum update httpd
...

PAM - Pluggable Authentication Modules for Linux
Linux-PAM Is a system of libraries that handle the authentication tasks
of applications (services) on the system. The library provides a sta-
ble general interface (Application Programming Interface - API) that
privilege granting programs (such as login(1) and su(1)) defer to to
perform standard authentication tasks
Linux-PAM separates the tasks of authentication into...