You may find this post informative. Here is the most relevant section:
What is a boot loader?
Most simply, a boot loader loads the operating system. When your
machine loads its operating system, the BIOS reads the first 512 bytes
of your bootable media (which is known as the master boot record, or
MBR). You can store the boot record of only one operating system in a
single MBR, so a problem becomes apparent when you require multiple
operating systems. Hence the need for more flexible boot loaders.
The master boot record itself holds two things -- either some of or
all of the boot loader program and the partition table (which holds
information regarding how the rest of the media is split up into
partitions). When the BIOS loads, it looks for data stored in the
first sector of the hard drive, the MBR; using the data stored in the
MBR, the BIOS activates the boot loader.
Due to the very small amount of data the BIOS can access, most boot
loaders load in two stages. In the first stage of the boot, the BIOS
loads a part of the boot loader known as the initial program loader,
or IPL. The IPL interrogates the partition table and subsequently is
able to load data wherever it may exist on the various media. This
action is used initially to locate the second stage boot loader, which
holds the remainder of the loader.
The second stage boot loader is the real meat of the boot loader; many
consider it the only real part of the boot loader. This contains the
more disk-intensive parts of the loader, such as user interfaces and
kernel loaders. These user interfaces can range from a simple command
line to the all-singing, all-dancing GUIs.
Boot loaders are usually configured in one of two ways: either as a
primary boot loader or as a secondary boot loader. Primary boot
loaders are where the first stage of the boot loader is installed on
the MBR (per the previous description). Secondary boot loaders are
where the first stage of the boot loader is installed onto a bootable
partition. A separate boot loader must then be installed into the MBR
and configured to pass control to the secondary boot loader.