Re: so many versions
There are two things, "Linux", which is the kernel itself and "GNU/Linux", which is the combination of the kernel and the userland (programs that the user runs).
Each distribution (Ubuntu, Debian, Red Hat) will take the Linux kernel source, maybe modify it with special sauce (note: NOT secret sauce, it's all out there), compile it, take user software packages, perhaps modify them with special sauce, compile them, package them all up in to bundles that can be installed, and provide an installer to install those packages.
Each distribution is responsible for providing packages of software, and backporting security fixes from that package. Each distribution makes its own choice about that; things like Gentoo will simply provide the most recent version of a package, eg Apache httpd 2.2.29 is the most recent release of Apache httpd 2.2, and Gentoo will provide that.
Other distributions, eg Red Hat Enterprise Linux, will have chosen a specific version of Apache httpd 2.2 to stick to (in RHEL 6, it is 2.2.15), and each time a new version of Apache is released, Red Hat engineers merge back any bug fixes and security fixes back to that version and release a new package.
The contrast to this is the BSD approach. Each of the BSD projects provides at least two things, a kernel and a "world". All of the software is "owned" by the BSD project, even when it is actually "vendor" code (code taken from elsewhere), and is developed and distributed in sync with each other. You can take a single export of source code of FreeBSD and build the entire kernel, userland, installer etc. There is more of a distinction between OS code, and 3rd party code.
Eg, gzip is part of the OS in FreeBSD. The program binary lives in /usr/bin/gzip, its source code can be found in /usr/src/usr.bin/gzip, and it is maintained by the FreeBSD project team. Apache httpd is not part of the OS, you must install a package to get it, and the binary lives in /usr/local/sbin/httpd.
On RHEL, both gzip and httpd are packages you can install (gzip being installed by default) or remove, no different to any other program.