An icon theme is a named set of icons. It is used to map from an iconname and size to a file. Themes may inherit from other themes as a way to extend them.
An icon file is an image that can be loaded and used as an icon. The supported image file formats are PNG, XPM and SVG. PNG is the recommended bitmap format, and SVG is for vectorized icons. XPM is supported due to backwards compability reasons, and it is not recommended that new themes use XPM files. Support for SVGs is optional.
Icons and themes are searched for in a set of directories, called base directories. The themes are stored in subdirectories of the base directories.