What the differences between a Linux or UNIX software update and an upgrade? How do I perform the both?
A software upgrade means a major change in version of a Linux or UNIX operating systems. Almost every part of the system is upgraded to the latest version. You need to purchase a software upgrade. For example, Redhat Enterprise Linux v5.0 can be upgraded to Redhat Enterprise Linux v6.0. You need to purchase RHEL v6.0 subscription using the online store or from retailers or from server manufacturers such as Dell, HP, Oracle etc. In most cases you need to reload the operating system and restore the data from the backups. Some operating systems such as FreeBSD or Debian / Ubuntu can be upgraded over the Internet itself using command line or GUI tools. If upgrade caused some sort of problems, you can always issue restore using backups. There is no roll-backups options available for the operating system upgrades.
A software update is nothing but updates (or patches) applied to your current version of software. Usually, all software updates are made available through the Internet. For example, Redhat Enterprise Linux v5.0 updates are avilable via RHN server. All software updates provides patches that will fix the security of your Linux / UNIX and can increase stability or compatibility to existing software. You need to apply patches or updates using the apt-get (Debian / Ubuntu and friends), yum or up2date (RHEL / Fedora / CentOS and friends) or portsnap & portmanager/portmaster (FreeBSD) commands or the using Apple menu >Software Update under Apple UNIX Mac OS X. In most cases version number is increased with each update. If update caused some sort of problems, you can always issue the rollback commands to go back to the older version.
Make a backup
It cannot be stressed enough how important it is to make a backup of your system before you perform upgrade or apple update. Full backups are recommend using the standard UNIX tools such as tar, rsync, rsnapshot and so on.