kexts are mainly hardware drivers. The big thing is they are loaded on-demand during boot usually (tho can sometimes be loaded or unloaded on the fly) but if the hardware claims the computer REQUIRES support for a piece of hardware, and no kext will claim it, you'll get a panic with "no supported drivers found for platform xxx".
If the driver isn't found but not essential, something won't work. Could be fan management (fans ramp up) or camera or mic don't work, etc. SOMETIMES you can copy kexts from one computer to another, but they are often OS version specific.
You'll run into both problems when trying to use say, a 10.6.3 retail install disc on a somewhat newer mac that shipped with say, 10.6.6. The restore disc may not be able to boot, and you have to boot the mac in FWTDM and cable it to an older machine that CAN boot the 10.6.3 disc, install it over firewire, then get the 10.6.8 combo run on it before it will boot on its own. The combo updaters tend to have the catch-up kexts and will clear up problems.
Another example, some of the early macbooks would boot off the 10.5.6 restore disc but the fans and wifi wouldn't work right after booting, until the combo updater was run.
Sometimes the combo updater won't do it. One line of imacs wouldn't have a functional camera even after the combo updater was run, you had to copy over the kext. But I've tried doing things like that recently and have gotten error messages telling me there was a kext with improper security settings or that wasn't installed properly etc, and it refused to use it. Not sure what's up with that.
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