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kernel of wisdom
What's "kernel patching?" Who needs to get this down?
Linux Help - Kernel Patching Guide
Most people have a fairly recent kernel. But since the kernel is constantly being updated, people on modems (such as myself) don't like downloading the whole source everytime a new version of the kernel comes out... It is a pain to download 14+ megs of stuff when 95% of it is the same stuff that you already have in your kernel source directory.
For this reason, kernel patches are released. Kernel patches contain only the files that have changed since the last kernel, hence making it less of a pain to upgrade.