An ANI file is a cursor animation file used by the Windows operating system. It contains image frames for the animation sequence as a collection of .ICO files. ANI files are similar to .CUR files, but contain multiple images used for animation.
ANI files are used to store default Windows animated cursors, such as the spinning hourglass. They can also be used for storing custom animated cursors. ANI files can be downloaded from third-party developers and are often included with custom desktop themes.
NOTE: On March 31, 2007, Microsoft issued a Security Advisory regarding a vulnerability in the way Windows handles animated cursor files. The issue affects Windows 2000 through Windows Vista.