The iCM Media Library holds all of the media items, images and downloads used by your site.
iCM organises media items into groups which can contain any media items of any type - it's up to you how you organise things. The security you add to groups controls who has access to the media items within them.
Every media item has a unique ID, and will be of a type defined by an iCM administrator who can set up media types using media definitions.
Access to the media library is controlled a user's Content Privileges.