Well known developers from around the world including Chainfire, Xplodwild and Dees_Troy have come together to build another aftermarket ROM (or custom ROM) which aims to make the Android operating system more open and user-friendly – the OmniROM.
We’ve all been following  the progress of CyanogenMod which by far is the most popular aftermarket ROM for Android devices with more than a handful of devices have official support, and innumerable ones having unofficial ports. The makers of CyanogenMod recently formed a company out of it and got themselves a hardware partner in OPPO. All this makes the birth of another promising aftermarket ROM — OmniROM — a very digestible story.
The ROM will be available for dev-friendly devices like the Nexus lineup (most devices, including the Nexus 4), OPPO Find 5, Xperia Z, Xperia Z Ultra, et al. Developers of  the ROM would hope to rival other popular ROMs such as CyanogemMod itself, Paranoid Android, AOKP, etc. OmniInstaller, an open-source ROM installer is also being worked upon, which will aim to make installation of any custom ROM easier on your Android smartphone. While the CyanogenMod team too is working on an installer, it is limited to CyanogenMod ROMs only.
What is rom?
ROM is all the system files of your phone. Try this question at Google.
This is only going to be on single sim phones as mediatek still won’t release source code and frameworks to allow any custom firmware like this or cyanogen on MK chip phones.