The core packages include the packages that are absolutely essential to the system’s functionality (such as the Linux Kernel, Binutils, Coreutils, GCC, etc) and any packages where upgrading to a new version has a high potential of breaking existing packages, configurations and other stuff (such as Apache, OpenSSL, PHP, Python, Perl, etc).
In practice, it boils down to the following being classified as core packages: the kernel, toolchain, scripting language interpreters, daemons, essential libraries and essential utilities.
The following packages are considered to be core packages (as of Cucumber Linux 1.1). They are classified by which category they fall into.