nexus s overclocking rooting?

i want to overclock my nexus s. what are exactly the steps? first install some kernel, root it or what?
and what exactly are the kernels? a user look-alike android system? can you link me a site that explains all these. Is it safe to root or install a kernel to my phone? can i return it to its first state so i won't lose guarrantee? is the difference between first and overclocked state perceptible, like in browsing? and how about the battery duration? thank you in advance