helper or Library

[eluser]crumpet[/eluser]
a helper is just a collection of global functions
a library is a class

Libraries work in an object oriented way. Helpers generally just save you code
I will do an example comparing a form library and a form helper

Library Functions
set_fields(array $fields) //set all the fields in the form
set_action($action) //set the forms action
display() //prints out hte form based on the other functions you used

Helper
form_open($action) //prints a <form> tag with the action you want
form_input($name) //prints an input tag

hopefully you can see the difference