Directories in UNIX form a tree structure, and all accessible files, even if they reside on other disks or in a network-mounted remote filesystem, are placed in that tree rather than being distinguished by "drive letters" or disk names.
Directories essentially contain lists of names, and are actually a special kind of file managed by the operating system (some UNIX versions allow you to read the binary contents, but you typically can't modify the contents directly).
All directories contain . (current) and .. (parent) entries that can be used for relative navigation. For example, ./name refers to the same file as name in the current directory, or if dir is the name of the current directory, ../dir/name refers to the same file as name.