Clarification on database access in helpers, models and plugins

[eluser]Ki[/eluser]
Gon,
Thanks for the reply - I agree with you, it is a bit overbloatd and I am trying to put it on a diet.
Can someone clarify the difference between Library, Helper, Model, Plugin in CI?
As far as I know:
Helper provides a group of functions
Model allows DB access and manipulation
Plugin is just one specific function
Where does the libary fits? How do you know when should you go with a combination of helpers and when to go with a library?