What is .iso file format. how to convert this into an image understandable by nero.?

I have a .iso file and want to covert it into a nero image. so that i can write it to a cd. because i have a .iso file of 699mb and i burned it directly into a empty disc. but it is useless as it didn't become a bootable linux OS cd. so please guide me to make the correct one. thank you!!!