When the CPU is in kernel mode, it is assumed to be executing trusted software, and thus it can execute any instructions and reference any memory addresses (i.e., locations in memory).The kernel (which is the core of the operating system and has complete control over everything that occurs in the system) is trusted software, but all other programs are considered untrusted software.In fact, some distributions are distinctly different down to the type of file types they use for package management.As you can see there are number of possible systems (and the above list is not even close to being all-inclusive).
Of course, at the time, most everyone trying their hand at Linux knew they were getting into something that would require some work. But even though Linux has evolved into the user-friendly operating system it is, there are still some systems that are fundamentally different than their Windows counterparts.
Thus, all user mode software must request use of the kernel by means of a system call in order to perform privileged instructions, such as process creation or input/output operations.
A system call is a request to the kernel in a Unix-like operating system by an active process for a service performed by the kernel. An active process is a process that is currently advancing in the CPU (while other processes are waiting in memory for their turns to use the CPU).
In this article 'How To Update Ubuntu Kernel' example is showed for Kernel 2.6.39-0.
If you want to know list of Kernel releases for Ubuntu, then you should to go the site lake are List or Check Installed Linux Kernels. There you will be able to see Kernels for other Linux distributions also, but to see for Ubuntu, just scroll page down.
So it is always best to understand those systems in order to be able to properly use those system.