A stock ROM is the version of the phone's operating system that comes with your phone when you buy it.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 a so called Devs in their thread or development sections.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.
Before installing any Custom ROM
You need to root your device for Superuser access to install or flash any custom ROM.
Your device should have at least 70% charge with it, to assure that there will be no Power interruption during the process.
Backup all essential Data’s on your phone.
Disable all security programs like Antivirus and Firewall from your computer and uninstall the same from your device, as these security suites generally tend to interrupt the process.
Enable USB debugging in your Samsung Galaxy Y Duos from the path “Settings >Applications > Development > USB debugging”. The option of USB Debugging should be ticked.