Custom Android Roms
Custom firmware is that which has been created by someone other than the official developer, usually adding extended functionality to the device that would not have been available otherwise. It is important to note that with most devices the warranty will become void in the event that the user installs custom firmware. If you are forced to return the device for whatever reason, it would be wise to re-flash the original official firmware before submitting it.
Please also note that there is a possibility you may brick your device should you pull the SD card / remove power while the firmware is being flashed.