What is a Custom ROM ?!

What Exactly is a Custom ROM ?

Hey People this is my first post, I don't wanna rush up directly to Apps,ROM's,Kernels,etc.
First let us know what are they exactly :)

ROM

Stock ROM- It is the version of the phone's ORIGINAL operating system that comes with your phone when you buy it.
Custom ROM- It is a fully standalone version of the OS (which makes everything run), apps, services, etc - everything you need to operate the device, except it's customized by someone in some way, which are lot interesting ;)
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 Prefer Custom ROMs

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.
Custom ROMs are oftentimes faster, more efficient, and use less memory because
The developer Remove 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.
The Problems of Custom ROMs

First of all, something may go wrong with the flashing process (that's the process of installing the ROM) and leave your phone in a bricked state. The chances of this are pretty low nowadays, and most of the time you can restore it back to normal.
Custom ROMs could have bugs… but then so do the stock ones.
However, in case you do find a bug, you actually have a 2-way channel of reporting it - post in the ROM forum and you will more than likely get an answer back and your bug acknowledged.