Well linux doesn't have C: and D: and G: and all that shit...it's basically got / and then mount points.
/ is called either "root" or the "root filesystem" to differentiate it from /root directory or the root user
Any folder or directory in a linux system can either have data like a windows folder like C:\windows\system32 on a linux system might be something like /usr/lib OR in linux a folder can serve as a mount point for another device.

so / will have folders like
/boot
/home
/etc
/var
and so on, but they may not all be stored on the same disk partition.

So if you split the root filesystem into 3 partitions you gain a few options.  You know how in windows if you reformat you lose all your settings and data?  Well if you mount another disk partition on /home in linux, and reformat / to rebuild the system after some catastrophic crash or something, all your user settings and all you data will still be there with no extra effort since it's a different disk partition.  Same as /boot, if the filesystem on C:\ in windows gets corrupted you can't boot ANYTHING, you're pooned and need to boot off a CD and checkdisk.  since the /boot file system with the kernel is separate you can usually get SOMETHING running and fsck the partition, also if you wipe your system you boot options are safe.

You can break out other folders like /etc to keep system wide settings safe.  etc