ROM stands for 'read only memory' this is all the information, code etc, stored on your hard drive. When you boot up some of this code is 'read' from your hard drive and 'written' into your RAM (ramdom access memory - the ram cards installed in your motherboard's memory slots). During the boot up process the first code loaded into RAM will be your operating system (such as Windows). When you launch an application (such as MS Word) this is also loaded into RAM. When you create documents with an application you are, essentially, re-writing (over-writing) that application's code that has been written to Ram (the source code on your disk is unaffected). This is why it is necessary to allow your computer to shut itself down - simply turning it off forces the computer to dump whatever is loaded into RAM back onto your Rom drive - which causes the drive to become 'fragmented'.