In the event it's not blindingly obvious, mounting.
Discuss what mounting is, why it's useful, removeable media (umount your floppies and flash sticks!), '/etc/fstab', '/proc/mounts', and '/etc/mtab'.
Please add text about network filesystems and loopback devices.
And about udev and how removable devices are mounted on a desktop system.
Feel free to fix/add/subtract:
Use "man mount" and "man umount" to see the syntax and options.
mount takes the raw block device file, usually a partition or a removable disk, and a destination directory called a mountpoint and makes the device available through the filesystem at the mountpoint so it can be conveniently accessed through the normal file access methods.
For example, "mount /dev/hda1 /mnt/foo" will try to mount the first partition on your primary master IDE drive so it appears at /mnt/foo/. Some guides may tell you to mount devices directly to /mnt, but this is not very convenient as you probably often want to mount more than one device at a time. You may wish to make mountpoints (regular directories, that is, so use mkdir) for commonly used devices and perhaps a dir such as /mnt/test for temporary/testing mounts.
If you find yourself running mount commands every time you start your system up you might consider opening up /etc/fstab (short name for "filesystem table") in your favorite available text editor (after you do a "man fstab" so you're not so likely to screw up and try to sue me for my lousy directions) so you can add the parameters and destinations for whatever you want mounted on startup.
There are also at least two automounters that can mount devices "on demand".