If you’ve rooted your Android device and have been messing around with third-party ROM replacements, you most likely have run across the mention of CPU governors and IO schedulers. The Linux kernel which is loaded on the ROM (which can often be independent of the ROM itself) control these. In essence, they dictate what method the CPU uses the ramp up and down (between the minimum and maximum frequencies) depending on workload, as well as the method that the OS uses for disk operations.
These changes can have a remarkable effect on device speed and battery life, but can be intimidating due to the number of options available. This guide provides some good insight into what the different options do. In combination with overclocking options, the options are plentiful.