Plugins – I put all 3rd party codes I’m using in my application as Plugins. I would, as best as I can, try to use classes rather than straight function calls.
Helpers – Any standalone straight functions calls, which are repetitive in nature, I classify them as Helpers. For example, sorting functions, my own calculation functions, etc
Libraries – I classify my own classes as ‘Libraries’. Normally, if I’m already writing a class in my application, it would then to be the core logic of the application, as such, I group them all in the Library folder.