Now, let's discuss the concept of directories. A directory is a collection of files. It can be thought of as a "folder" that contains many different files. Directories are given names, with which they can be identified. Furthermore, directories are maintained in a tree-like structure; that is, directories may contain other directories. The top level directory is called the "root directory" and denoted by /; it contains all the files in your system.
A pathname is a file's "full name"; it contains not only filename but also its location. It is made up of the filename, preceded by the name of the directory containing that file. This, in turn, is preceded by the name of directory containing that directory, and so on. A typical pathname may look like /home/sasha/talk.txt which refers to the file talk.txt in the directory sasha which in turn is a subdirectory in /home.
As you can see, the directory and filename are separated by a single slash (/). For this reason, filenames themselves cannot contain the / character. MS-DOS users will find this convention familiar, although in the MS-DOS world the backslash (\) is used instead. The directory that contains a given subdirectory is known as the parent directory. Here, the directory home is the parent of the directory sasha.
Each user has a home directory, which is the directory set aside for that user to store his or her files. Usually, user home directories are contained under /home, and are named for the user owning that directory, so that the home directory of user sasha would be /home/sasha.
At any moment, commands that you enter are assumed to be relative to your current working directory. You can think of your working directory as the directory in which you are currently "located". When you first log in, your working directory is set to your home directory — for user sasha, it would be /home/sasha. Whenever you refer to a file, you may refer to it in relationship to your current working directory, rather than specifying the full pathname of the file.
For example, if your current directory is /home/sasha, and you have a file there called talk.txt, you can refer to it just by its file name: a command like emacs talk.txt issued from the directory /home/sasha is equivalent to emacs /home/sasha/talk.txt (emacs is an extremely powerful editor for text files; new users may prefer something simpler, such as gnotepad, but for power user, emacs is indispensable).
Similarly, if, in /home/sasha you have a subdirectory called papers and, in that subdirectory, a file called fieldtheory.txt, you can refer to it as papers/fieldtheory.txt.
If you begin a filename (like papers/fieldtheory.txt) with a character other than /, you're referring to the file in terms relative to your current working directory. This is known as a relative pathname. On the other hand, if you begin a filename with a /, the system interprets this as a full pathname — that is, a pathname that includes the entire path to the file, starting from the root directory, /. Use of the full pathname is known as an absolute pathname.
Here are some standard conventions you can use in paths:
~/ — user's home directory
./ — current working directory
../ — parent of the current directory
For example, if sasha's current directory is /home/sasha/papers, he can refer to the file /home/sasha/talk.txt as ~/talk.txt or as ../talk.txt.