I'M not too good at explaining, but found this. Hope its easy to understand.
The MBR is the Master Boot Record. It is a (typically) little partition of your harddrive
that holds a boot loader. The MBR is the partition your computer boots into when it starts up.
A boot loader is an application that passes control from the MBR to an operating system located on another partition.
So, in short, your computer boots like so:
2. Master Boot Record partition loads the boot loader
3. User selects an OS (or boot loader times out and default OS is selected
4. OS loads
Would this help maybe?