In the simplest case, your entire Unix file system lives in just one
disk partition. While you'll see this arrangement on some small personal
Unix systems, it's unusual. More typical is for it to be spread across
several disk partitions, possibly on different physical disks. So, for
example, your system may have one small partition where the kernel lives, a
slightly larger one where OS utilities live, and a much bigger one where
user home directories live.
The only partition you'll have access to immediately after system
boot is your root partition,
which is (almost always) the one you booted from. It holds the root
directory of the file system, the top node from which everything else
hangs.
The other partitions in the system have to be attached to this root
in order for your entire, multiple-partition file system to be accessible.
About midway through the boot process, your Unix will make these non-root
partitions accessible. It will
mount
each one onto a directory on the root partition.
For example, if you have a Unix directory called `/usr', it is probably
a mount point to a partition that contains many programs installed with
your Unix but not required during initial boot.
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