What does it mean to install a kernel?

Question

What does it mean to install a new version of the kernel. My Linux box gave me this message when I was updating,

NOTE, 3.8.13 was the last maintained maintenance release by Greg Kroah-Hartman. It is recommend to move on to linux310-series.

What I want to know is,

• Is it really that simple to only change the Linux kernel?
• Is the Linux kernel like a simple executable file that can be swapped out for another Linux kernel?
• What happens if I install a new kernel while the box is already running another version of the kernel?
• Right now I'm using Kernel 3.8.13.8-1. Is it really okay to move to the linux310-series as the above update message says?

Answer 1

• Yes.
• Yes, sort of. It's not simple and it's not "just" an executable file, but you can swap between Linux kernels super easily. This is not applicable to every kernel, though, meaning that e.g. you can probably replace a FreeBSD kernel with a newer FreeBSD kernel, and as stated above, you can upgrade a Linux kernel pretty easily, but you cannot easily replace a Linux kernel with a FreeBSD kernel. See the Wikipedia page on kernels. I'd link you to the "what is a kernel" question, but I can't find it.
• Your box will continue to use your current kernel. It depends on your bootloader and distribution, but when you reboot, you'll probably be able to choose between the two kernels.
• Probably. Upgrading your kernel isn't actually a big deal, especially since (if you're smart, and keep your old kernel around just in case) you can go back to the old one if the new kernel doesn't work for some reason. Sometimes there are problems with new kernel versions, especially if you have exotic hardware, but I've never seen any myself.

Answer 2

What box is this? What Linux distribution? Where do the "updates" you mention come from?

You are certainly wellcome to play around, and install the latest (stable) kernel version (or even a bleeding edge -rcX one from Linus), but as mentioned above this might mean some hardware isn't supported anymore (very unlikely), some hardware support has changed and might now break (also unlikely, but not unheard of). Sometimes changes in the kernel force changes in the ancilliary software (tools for configuring and so on the system), but that shouldn't be an issue here. In any case, it is required to know a bit of what you are doing, so plan to spend quite some time reading up, defining a correct kernel configuration (there are literally hundreds of knobs to frob!), building and checking everything works. Start with the configuration of your distribution's kernel.

It is easier (and safer) to depend on the experts for your distribution do the job properly. But certainly not so much fun (where is the fun if each boot goes boringly the same routine, instead of blowing up or giving a firework display?).