The kernel is the heart and soul of the operating system. All applications, utilities, etc. call on some part of the kernel to do whatever it is that they do.
To make a smaller kernel (and take up less space on the hard drive), some parts of the kernel can be compiled as modules which can be loaded and unloaded as needed.
The Linux kernel HOWTO can tell you about it in much more detail. Google it up at
www.google.com/linux.
If the distros you want to install are recent versions, it's likely they all have the 2.6 kernel.
What should you do? Go for it! If you have difficulties, come back with specific problems and as much information as you can provide in re the problems.