Linux is a Kernel
A Kernel is a part of an operating system.
The kernel resides in memory. Memory allocation for kernels is usually separate from user-driven content.
The kernel facilitates interaction between software and hardware. Things like memory (ram), storage (the hard drive), and networking (hard-wired or wireless internet) are "handled" by the kernel.
There are many versions, many distributions
The kernel manages some super "low-level" computing details - "...booting, memory, processes, drivers, and other lowest-level concerns of the operating system...".
Ubuntu
Ubuntu is one of the many distros. Ubuntu will be the focus in these linux write-ups.
Ubuntu is based on the Debian distro of linux.