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As you can see, the Linux filesystem resembles a tree, with a main branch that 
extends outward and directories branch out from one another. This default directory 
structure is part of the 
Filesystem Hierarchy Standard
 (
FHS
), which is a set of 
guidelines that defines how the directory structure is laid out. This specification 
defines the names of the directories, where they are located, and what they are for. 
Distributions will sometimes go against some of the definitions here, but for the most 
part, follow it fairly closely. This is why you may see a very similar (if not the same) 
directory structure on distributions of Linux other than Ubuntu.
So, why is 
this important? As I mentioned, each directory generally has a purpose. 
There are some debates about the default layout from time to time, and some 
changes are made every now and then. But in regard to Linux, the filesystem layout 
tends to change less frequently than other things do.
A full walkthrough of the FHS is beyond the scope of this book, but I have included 
a link to this specification at the end of the chapter should you decide to read more 
about it. There are some directories you definitely should know, however. Here are 
some of the more important ones.