Nowadays, a UNIX system is big, very big. This is especially true of GNU/Linux: the amount of software available would make for an unmanageable system if there weren't any guidelines for the location of files in the tree.
The acknowledged standard is the FHS (Filesystem Hierarchy Standard), which was at version 2.3 at press time. The document which describes the standard is available on the Internet in different formats on The Pathname web site. This chapter will only provide a brief summary, but it should be enough to show you which directory is likely to contain a given file, or where a given file should be placed.