Grub stands for Grand Universal Boot Loader. For what that's worth.
Your BIOS after checking out the hardware sends the heads of your boot disk to track one sector one, and does a read. This is where the MGR ( Master Boot Record ) is. The MBR directs operations from there. The MBR is an executable program, it takes control of the system, and goes and looks for the operating system files to load. ( Generic explanation ). This happens regardless of the OS installed.
Windoze writes its own MBR record when installed, and it will load only windoze. If you install a linux system, Lilo and Grub are the most common options to use. Either can be installed on the MBR, and therefore overwriting the windoze MBR.
During a linux install, the system will look for any other OS's, including windoze, and add a record to the menu, so you can choose to boot windoze, or linux. You can install multiple OS's and boot any one of them by adding the correct information to Lilo or Grub. If given a choice, pick Grub. It is ( in my opinion ) easier to maintain. Lilo and Grub have all the code you need to load most OS's.
Good advice above, back up any windoze stuff before begin installing linux. If you mess up the MBR, it can be recovered later. There are many ways to do that.
If you are going to multi boot, Windoze systems should be installed first. Then install what ever linux system you choose later. If you try it the other way around, the windoze install will re-write the MBR, clobber the Lilo or Grub code, and only windoze will boot. That is the way windoze is designed.
Hope this clarifies some things for you.