Where is the bootloader stored - in ROM, RAM, or elsewhere?

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http://askubuntu.com – According to the wikipedia entry, a bootloader is a small program stored in ROM(a part of the main memory(RAM), isn't it??) which can only be read and not erased. I am a little puzzled here. Does this mean that every RAM we buy has a bootloader installed by default? And also, I read from a book which says that there is typically an area called MBR on the hard drive which includes a bootloader... So where exactly does this bootloader locate? Do different OS store their bootloader in different places? (HowTos)