QUOTE(girl83 @ Mar 20 2013, 03:43 PM)
hmm, to do custom ROMs what is the benefits and what is ROMs actually? should put in first post for newbie like me to learn
A custom ROM is a fully standalone version of the OS, including the kernel (which makes everything run), apps, services, etc - everything you need to operate the device, except it's customized by someone in some way.
So what does the "customized" part mean?
Since Android is open source, developers are free to take stock ROMs, modify them, strip them of garbage, optimize them, add things, and pretty much do whatever their imagination and skills allow.
Why You Want Custom ROMs?

1.Update Frequency
Using a custom ROM usually results in more frequent updates that fix bugs and introduce new features because the developer behind the ROM doesn't have the same procedures and red tape that the manufacturer+carrier combo does.
A quality update can be churned faster because it doesn't involve the bureaucracy of 30 different project managers, 15 vice presidents, and 5 dozen marketing departments.
A ROM developer usually gains a loyal community which beta tests his updates in real life situations and provide feedback, or even fixes bugs - that's the beauty of open source software.
Oh and did I mention that ROMs are usually free and supported by optional donations? If you can't afford to pay for it, you don't have to.
Finally, most custom ROMs out there are updateable over the air (OTA) and without reinstalling anything.
Who doesn't love open source after this?
2.Better Performance And Efficiency
Custom ROMs are oftentimes faster, more efficient, and use less memory because
the developer ripped out useless garbage, such as carrier installed apps or
the developer optimized the kernel. For example, an undervolted kernel can provide a much better battery life than the stock one.
3.Upgrading To A Better/Later Version Of Android
You can upgrade to a version of the OS that has not yet been released for your device, or never will be. This is possible in 2 situations:
A leaked version of the new ROM showed up online, and the developer got on it like the fat kid chasing an ice cream truck. This was exactly the case with Hero and Android 2.1, allowing me to upgrade from 1.5 weeks (months) before the official version was available and ditch the bogged down and slow stock OS.
A ROM from another phone was ported by the developer to work on yours. For example, G1 and MyTouch 3G users may never see Android 2.1 officially released on their phones, but Cyanogen, one of the most respected Android developers, was recently able to create a custom ROM running 2.1 for those devices. Oh, what a happy day it was for MT3G and G1 owners.
4.Ability To Install Apps To The SD Card
Most custom ROMs nowadays come with the ability to install applications to the SD card, called Apps2SD (or A2SD).
This is currently not possible on stock ROMs, even in Android 2.1 and is supposedly on Google's TODO list.
If you have run out of space on your phone (which I have repeatedly on my Hero), Apps2SD is a killer feature to have.
This post has been edited by mudkipryan94: Mar 20 2013, 03:49 PM