One area that many people new to Linux find confusing is the matter of how partitions are used and accessed by the Linux operating system. In Windows or MS-DOS, the naming scheme is relatively simple: each partition gets a drive letter. You then use the correct drive letter to refer to files and directories on its corresponding partition.
This is entirely different from how Linux deals with partitions and, for that matter, with disk storage in general. The main difference is that each partition is used to form part of the storage necessary to support a single set of files and directories. This is done by associating a partition with a directory through a process known as mounting. Mounting a partition makes its storage available starting at the specified directory (known as a mount point).
For example, if partition /dev/hda5 were mounted on /usr, that would mean that all files and directories under /usr would physically reside on /dev/hda5. So the file /usr/share/doc/FAQ/txt/Linux-FAQ would be stored on /dev/hda5, while the file /etc/X11/gdm/Sessions/Gnome would not.
Continuing our example, it is also possible that one or more directories below /usr would be mount points for other partitions. For instance, a partition (say, /dev/hda7) could be mounted on /usr/local, meaning that /usr/local/man/whatis would then reside on /dev/hda7 rather than /dev/hda5.