Library vs. Folder vs. Label vs. Albulm

What is the difference between a library and a folder?
A library is a grouping of items that could be files, emails, or data that is independent of the file system.  A folder or directory is usually part of the file system (although it is sometimes a virtual folder used in a library which is a different type).
Examples of libraries: iPhoto and iTunes uses a library system.  Macintosh also has a separate library that groups files together by category.
iPhoto library is put into it’s own container, so people do not accidentally mess with the library. People can make albums from different parts of the library, and then it does not create a whole new directory or files- it actually just links to them.
Windows 7 photo library may group emails that have photos along with photo files. To delete the emails you still have to delete them from the email program.
Picasa also uses a library, but is still primarily based on the file system.
Labels vs. Folders
The only place I know that uses labels is GMail or Google mail, but it is important to know since many people use GMail or Google Mail.  An email can have more than one label, but cannot be in more than one folder.  Like an album in iPhoto or iTunes a label is just a link or tag for easy categorizing.
Database System vs File Folder System
A database reduces redundancy, disk space, and time.  Databases can store large amounts of information without taking as much room as files.  It is also easier to find things in a database.  Many of today’s web sites are actually databases that are part of a content management system.  Even though they may appear to have a file structure [ ie: ], that may actually be a hidden database redirect.