what are the differences between libraries, helpers and plugins in CI ?

[eluser]bretticus[/eluser]
Besides the manual that explains these all quite well...

libraries: Utility classes where object state is important (payment gateways, authentication, etc.)

helpers: Collections of related functions (not classes) that do repetitive tasks (strings, arrays, etc.)

plugins: A simple way to drop in third party classes. Typically, the whole process is called with a single wrapper function. (deprecated in the upcoming version 2.0 of CodeIgniter.)