Kernel
What is the kernel ?
The kernel is the software that directly manages your hardware, allowing application libraries and software like GNOME and Firefox to run on many types of hardware without much difficulty. Because the Linux kernel is the core component of a GNU/Linux system, when it is upgraded, a full restart is required.
Types
Ubuntu packages the Linux kernel for a variety of architectures, including several variants of the x86 architecture. These include a 386 version, a 686 version, and versions for the AMD k6 and k7 processors. While most software for x86 processors in Ubuntu is compiled for 386 or better instruction sets, the kernel and a few other packages are specifically compiled for certain processors for speed reasons. Check the package documentation to determine what type of kernel will perform best for your processor.
Versions
Ubuntu packages the latest 2.6 kernel for optimal desktop speed and features. However, if you want to use 2.4, you still can.
SMP
Some motherboards have more than one processor on them, and some processors have multiple cores. If your computer is like this, then the SMP kernel is for you. Non-SMP kernels will not be able to take advantage of your multiple processors. However, if you do not have multiple processors, the additional code in an SMP kernel will only slow you down. Naturally, Ubuntu provides both SMP and non-SMP kernels for all supported architectures.
HowTo Upgrade or Recompile Kernel
The precompiled kernels that are supplied with with your distro should be fine however if you wish to update or optimise (or standardise) for your platform :
You can UpgradeKernel easily using Ubuntu.
You can also Kernel/Compile it yourself from LinuxKernelSource or UbuntuKernelSource.