ATG tag library is like a jsp standard library. The only difference is that ATG framework has provides his own tag libraries like DSP/DSPEL/CORE. You can find this details in ATG documentation.
Droplet’s main use is to display the contents of jsp page. There are many inbuilt droplet provided by ATG.
Pipe-line-> It is like a normal servlet pipeline basic funda. A pipe line is a sequence of programs in queue to be executed.
Droplets main use in jsp page to dispay the contents dynamically.
Dynamo components scope-> By default the dynamo component scope is global. And mainly three types of scope you use in ATG->request,session,global depending on your requirements.