Octopus and Barracuda - Dual-Core Aegir Installer

Drupal Core is the Core files of Drupal.

If you look at the directory structure every thing that is not is the sites folder is Drupal Core.
In the sites folder you generally have:
- default settings and uploaded files
- a themes folder - for the themes you install
- a module folder - for contributed modules you install
- a libraries folder - for various libraries - eg Java

When you upload a new version of drupal you can delete the SITES folder in the new version and drop it on the old version. If all goes well you have completed an upgrade.

Things to note:
There is also a profiles dir, which if you installed a specific profile the files could be in there.
(and backup before upgrading as modules are not always compatible with the new version - although currently drupal(7) is quiet stable in this regard)

The "Dual-Core" was the old BOA jargon to emphasize the fact that there are two installers (Barracuda and Octopus) and while you could use them separately (like Barracuda without Octopus) they work the best in a tandem, hence the CPU with 2 Cores analogy, which has nothing to do with hardware it can be installed on.

Later we have even better glued these two installers together, because you just run a single meta-installer on initial install (the boa script), and later you can upgrade the system with barracuda script and the Aegir instances with the octopus script.