In Ubuntu, use dpkg -l to list all installed packages.
In OpenSUSE, use zypper pa | grep ^i to list all installed packages.
In Ubuntu, use apt-cache search PACKAGE, where PACKAGE is a pattern.
In OpenSUSE, use zypper search PACKAGE.
In CentOS, use yum search PACKAGE.
In Ubuntu, use apt-cache show PACKAGE. This will display the version and description of the package.
In Ubuntu, use apt-get install PACKAGE.
In CentOS, use yum install PACKAGE. You can use the -y option to answer "yes" to all questions.
In Ubuntu, use apt-get remove PACKAGE.
Out of date software can pose a security risk, but upgrading can also introduce bugs. Remember to backup your data and configuration before upgrading packages.
In Ubuntu, use apt-get update to retrieve a new list of packages. Then run apt-get upgrade to upgrade packages to newer versions. Some packages, like the kernel, won't be upgraded automatically. To update these run apt-get dist-upgrade instead.