how to integrate a web application with SAP R/3 using the SAP XI technology? how does the data transfer take place?

Web application can be integrated with SAP R/3 using Connectors(Ex java connectors which is supplied by SAP). The data frm Web application can be obtained in the from of XML format and this data can be fed into SAP XI where it supports the XML data. This whole integration can be done using the connectors readily available. you have to develop your own connectors/Adapters for this intergration depending on the requirement.