design philosophy: helpers,libraries, and bears oh my

[eluser]n0xie[/eluser]
This is how I see it.

Helpers
Standalone functions (or static classes) that change some output in some way. Either by re-ordering data, transforming data, or styling data.
Example: site_url() : adds the the domain name to an url if it's not there

Library
(Standalone) Classes that are usually not application specific.
Example: Message Library: a class that takes care of displaying messages to an user

Models
Data Abstraction Layer that is usually application specific
Example: Page_model: contains and encapsulates your page logic and a way to access the persistent store (usually a db)