What is a BIOS / BOOT loader ?
The BIOS is a program which is linked with the motherboard. It makes more sense to it when one knows its acronym "Basic Input Output System", so to say the BIOS is here in order to identify the different components which are plugged to the motherboard (hard drive, DVD reader...).
What is a BOOT loader?
That's the program launched just after the BIOS in order to indicate which partitions of your hard drive you would like to execute, so to say which operating system you would like to use as you can have several operating system on the same computer.