Where do libraries and helpers fit in MVC?

[eluser]Unknown[/eluser]
I'm new to CodeIgniter and MVC, and am working on my first app using the framework. I have done PHP for sometime though.

I am setting up a multiple site system where sites share the models directory via symlinks. They also share the core. Each site has its own controllers and views.

My question is when do I use/write something as a library instead of putting that logic into the model. Same question for helpers. When it is best to write a library/helper? What would some basic examples be?

Thanks,
David