Icons may be stored in "/.icons" or "/usr/share/icons". Icons that are in "/.icons" are only accessible to the owner of that home folder. While icons in "/usr/share/icons" are global-icons that are accessible by all users. "/usr/share/pixmaps" contains icons that are installed or used by various applications and software packages.
Applications that are looking for an icon will first look in "~/.icons". Next, the application will try "/usr/share/icons", and then "/usr/share/pixmaps".
An icon theme has a specific structure/format that must be used to create a usable icon-pack. Inside the icon directories ("~/.icons" and "/usr/share/icons"), each icon theme has its own folder that is named after the icon-theme. Within the icon-theme's directory are several folders and some important files. For instance, an icon-theme may contain the below listed directories and files.