What are updates, upgrades, and errata?
Oracle (Enterprise) Linux and the distro it is derived from have several ways of releasing new versions of packages.
EL4 and EL5 are major releases.
For each major release, OS updates are released that can be applied to an existing system to apply bug fixes, add minor new functionality, or support new hardware. These updates are referred to in various ways. For example you may see reference to EL4 Update 8, or EL4U8, or EL4.8. They all mean the same thing.
Between updates, important bug fixes are released as "errata". These are individual packages with just a few bug fixes.
Going from one major release to the next is called an upgrade. At this time only a complete reinstall of the OS is supported when upgrading the OS. The new OS release is installed using the installation media (CD, DVD, or iso)
Going from one update level to the next is called an OS update. The update can be applied using installation media, or with the yum or up2date commands. Updates are available for free via http://public-yum.oracle.com/, and of course also via ULN.
Applying errata is called a patch or a package update. Errata are only available via ULN, and require an Oracle Linux support license.