In this tree, gilbert, goodyear , etc., are userenames. When you log on you are automatically put into your top directory, in my case, gilbert . When I login I am in my top directory. To refer to files from here I either type the filename, eg header.tex , or for files in other directories, I type the path and filename. Thus header.tex and notes.tex are both filenames , each having the file extension tex .
Subdirectories can go down several levels.
Most unix commands are described in the online manual, which is accessed by the command man , for example, try man date .