SOAP (Simple Object Access Protocol) is a transport protocol for sending and receiving requests and responses on XML format, which can be used on top of transport protocols such as HTTP, SMTP, UDP, etc

SOAP is XML based Web Service uses WSDL (Web Services Description Language)  for describing web services and how to access them in xml format.

SOAP follows JAXB (Java Architecture for XML Binding) standard  that defines how Java objects are converted from and to XML. It makes reading and writing of XML via Java relatively easy

SOAP uses JAX-WS (Java API for XML Web Serviceswhich is a set of APIs for creating web services in XML format.