An ICNS file is an icon file used by Mac OS X applications. It stores one or more images, typically which were originally .PNG files and supports 1-bit and 8-bit alpha channels. ICNS files are used for displaying the application icon in the Mac OS X Finder and in the OS X dock.
ICNS files are generally located within the Contents/Resources/ directory inside the application package. They are also referenced by the application's .PLIST file (Info.plist) using the CFBundleIconFile property.
Mac OS X icon files can be viewed with the Apple Preview program included with Mac OS X. They can also be created using Icon Composer, another utility included with Mac OS X. ICNS files support icon images of 16x16, 32x32, 48x48, 128x128, 256x256, and 512x512 pixels, as well as 1024x1024 pixels as of Mac OS X 10.8 Mountain Lion.