The original ESXi system provided by VMware is made up of multiple software packages (currently 60 for ESXi 5.0 and 65 for ESXi 5.1) called VIBs (VMware Installation Bundles). There is e.g. one large VIB for the base system (esx-base), one for the VMware Tools (tools-light), and most of the remaining VIBs are hardware device drivers (e.g. net-e1000 or sata-ahci).
With an ESXi patch one or multiple of these VIBs are updated (or in very rare cases added). An example: The latest patch for ESXi 5.0 (ESXi-5.0.0-201209001 of Sep 2012) updates the VIBsesx-base, tools-light and misc-drivers.

ESXi patches are provided by VMware in the format of so-called Offline Bundles (in this case also called Patch Bundles) in ZIP format.
Now the most important point to understand is that these Patch Bundles do not only include the patched/updated VIBs, but all VIBs that make up an ESXi system, and in fact the latest version of them all.

Read more about this at v-front.de