For example, your root file system / must exist on one 'volume' (partition). If you have /usr on another hard disk, you create an empty directory called /usr on the first hard disk, and you Mount the file system on the second disk to that mount point. You can then access the files in the /usr.
If you're unfamiliar with the standard Linux file layout, see FileSystemHierarchy.
Usually, when your system starts up, it will mount all the file systems (partitions) listed in /etc/fstab.
You can mount additional file systems using the mount command from the command line.
Note that if you mount a partition (filesystem) in a non-empty directory, Linux will ignore (but not damage) the files already in that directory; they will reappear if the partition is unmounted. However this is not usually done, it's normal practice to create an empty directory for the mount point.
Typically, you may wish to mount a USB flash card or card reader with e.g. mount -t vfat /dev/sda1 /mnt (where vfat is the file type, /dev/sda1 is where the USB card reader is plugged in, and /mnt is the mount point). Cards in some multi-card readers of the type that support SD, XD, MMC, CF etc cards may show up as /dev/sdb1, /dev/sdc1 or /dev/sdd1 instead).
Remember to un-mount the card (with e.g. umount /mnt ) before unplugging the USB connection. (Note it's 'umount' not 'unmount').
More recent Linux distros will save you the bother by auto-mounting CD's and USB cards.
This system is completely extensible - your filesystem can grow from / as far as it needs to grow.