What is SOAP - Simple Object Access Protocol

Web services are used to handle interoperability problem and have given a way to exchange the data in different platforms.

It uses Simple Object Access Protocol (SOAP) to transport data over networks. XML is widely used for structuring, coding data and decoding data. 

SOAP in short -
SOAP is XML based protocol, format-for-sending-messages and used over HTTP.

For more brief, please refer below link- http://searchsoa.techtarget.com/definition/SOAP
Whenever a service is build, we mainly consider two approaches to use -
  1. REST and
  2. SOAP.  
Note: SOAP is a protocol and REST is an architecture.
SOAP is slightly heavier, complex and give less performance than REST but it offers more options when security and flexibility is concerned.  SOAP comes with a complete specifications and widely used in customized fashion. SOAP envelope can have headers as well as body parts. This give you the option to use the full expressiveness of XML for sending and receiving out of band information.