Q1: What is the ROM or Firmware?
A1: ROM or Firmware is the place where the operating system that runs on our mobile phones are installed or in place. In case this is Google's Android Gingerbread version 2.3.
Simply put, if you change the ROM means you actually are doing the OS re-install into your phone.
Custom ROM's and then what? Custom ROMs are the Android OS is customized to meet the wishes of the creator of the Custom ROM. There ROM devoted to batteries, there is intended for performance.
so to select the ROM which is suitable for you, go back to yourself.
Q2: What is kernel?
A2: The kernel is the part of the phone that handles Drivers, Power Management, etc.. The kernel therefore indirectly affect the performance is quite large with the phone itself.
There are many custom kernel, custom kernel but not all fit with a custom ROM available.
Q3: What is root / rooting it?
A3: The Root is the user account in Linux (Android was developed based on Linux) that has any access. Root user can edit any of the system. For security reasons, the normal user does not have all the access in the system.