From Michael Amorose:
Carbon applications, the MacOS X Finder, and the Finder in MacOS 9.x use an 'icns' resource for defining an entire family of application icons. This includes all icon types from 'icl8' all the way up to Mac OS X's 128-pixel 'Thumbnail' icons used for display in the MacOS X Finder. Native MacOS X applications define this icon resource in a .icns file which can be edited by using the developer tools included with MacOS X. One annoying aspect of MacOS X development is trying to get the icon data from an .icns file into an 'icns' resource for use in your Carbon applications. icnsConverter easily solves this problem.