Android is made up of a small collection of "images", each with its own specific job. Last time we talked about the Recovery Image, today we're going to dig into the System Image — the funnest one of all!
What is the System Image?
In desktop and laptop computers, the System Image is basically the operating system (Windows, Mac, Linux, Unix, etc.) that you spend all your time in. You turn on your computer, wait while it boots, then run all your apps and do all your work on the desktop which lives inside the System Image. On Android it's not that dissimilar.
When Android boots up it takes you right into the System Image. Most users spend their lives running the "stock" System Image — the one that came pre-installed on their phone or tablet. That image contains all the settings, configurations, and apps that both the OEM and the carrier have decided are best for you. They update it whenever they get around to it (unless they have a new phone or tablet that they want you to buy). You can't change things that they don't want you to change — not easily.
Unless you go with a custom ROM...
In addition to the operating system itself, the System Image contains wallpapers, sounds, icon images, configuration and property files, the kernel, and a whole bunch of apps.
When you flash a custom ROM like CyanogenMod or AOKP, you're essentially replacing your System Image with a new one with all the customizations, settings, and apps that the ROM authors have put spent tireless hours crafting into their custom ROM.
These custom ROMs are usually kept more up-to-date than their stock counterparts, and typically run faster and more smoothly. Most have fewer pre-installed apps (which we affectionately refer to as Bloatware), so you have more room to put the stuff you want (movies, music, apps, wallpapers, etc.).