Confusion regarding helper plugin and library

[eluser]UdayD[/eluser]
I want develop a site using CI. But a bit confused between a plugin helper and library.

Where i should place what?
Like suppose i have a function to validate a user which i will require in all the files. where should i place it in library or create a helper for it or where so that i can use it in all my controllers.

Similarly whats the difference between a helper and plugin.

[eluser]MWebber[/eluser]
Helpers are collection of function(methods)
Plugin is only one function(method)

The main difference is that a plugin usually provides a single function, whereas a Helper is usually a collection of functions. Helpers are also considered a part of the core system; plugins are intended to be created and shared by our community.

(from the user guide)

[eluser]InsiteFX[/eluser]
As of CodeIgniter 2.0 there will be no more Plugins!

InsiteFX