ROM's are majorly Deadened.. They consist of some moded features from different phone or version of Android.. They can be modified as required..
Kernel's consist of CPU & GPU governors, schedulers, CPU linking, CPU & GPU overclocking, etc..
You can flash a rom without unlocking your bootloader, but you cannot flash a kernel without unlocking the bootloader..
Sent from my LT26i using xda app-developers app
Sony Xperia S LT26i
ROM: Trinity Vengeance RaptorXL™ HD 4.1
KERNEL: Trinity Ultimate Kernel