We frequently use some kind of web services to ensure communication between the front end web or mobile application, and the "invisible" back end application. (XML) messages are being transferred using HTTP between both applications, for example using the SOAP protocol.
In some cases, the interaction is mostly unidirectional. An example here is a B2B website in which a prospect might trigger a request to make a tender. This request is often redirected to a typical sales automation platform such as Salesforce.
In other cases, interaction is bidirectional. An e-commerce website might use product data collected from an ERP-system, but when an order is being made on the website, it will also be transferred to the ERP system for further processing.
The interaction of a new web application with existing systems and frameworks results in added returns or efficiency. However, this is often one of the more complex steps in a project. This is why a functional and technical analysis is of utmost importance to be able to make a plan of approach, including an estimated timing and budget.