Your application must use Plivo RESTXML to receive and control an incoming call. Some possible control actions could be: playing a music file while the caller is on hold, reading out specified text to the caller asking for inputs, dialing and connecting to another number.
Here, Plivo works like an HTTP Client which receives the call and requests your web application for instructions to control the call.
Your application must use Plivo RESTAPIs to initiate actions – such as making outbound calls, scheduling hang-up of calls, transferring calls etc.
Here, Plivo works like an HTTP Server which receives API requests from your web application, and executes the specified action(s).
The Plivo Helper Libraries provide a set of helper functions in a variety of programming languages to simplify the process of connecting your web application to Plivo. The Helper Library is by no means mandatory to interface with Plivo.