Generic File Icon
This is a generic file, that is, one that doesn't have any application ownership information stored in the Mac file system or its own resource fork. Opening these files typically results in the TeachText or SimpleText application being used.
Generic Folder Icon
This is a standard folder icon on the Macintosh. Folders can contain just about anything, including files, applications and other folders. Opening a folder results in the contents of that folder being displayed in a separate window on the Macintosh.
System Folder Icon
A special folder at the top-most level of the boot disk on the Macintosh is the System Folder. It contains all the files, applications, and information needed to run and maintain the Macintosh operating system itself. The "X" inside the folder icon indicates that this particular System Folder is live and that the information inside was used to actually start up the current Macintosh.
All of the major applications in Mac OS X live in a shared folder called the Applications Folder. It's easily recognized by the 'A' on the folder icon itself and is the first place to look when you seek any of the many Macintosh applications included with the operating system.