SOAP stands for Simple Access protocol. SOAP was first offered in 1990's and has been introduced to W3C in 2000. The main purpose of SOAP to be able to send request to invoke programs on remote computers using XML wrappers.
SOAP (Simple Access Object Protocol) is a communication protocol which is platform and language independent. For example the client side code might have been written using C and the server side is say written in Java.Both client and server agree on how to interpret theSOAP message.
SOAP basically structures the message in XML before it could be used to transform the useful information between servers and clients.
The next question that comes to our mind is :Why XML is used to structure the SOAP message? The answer is pretty straight forward: Becasue of XML's open source nature and widely acceptance it is used by SOAP.
SOAP can use different transfer methods but the simple and the most common is HTTP...because of its universal acceptance(all browsers support HTTP).
HTTPS is also as the transport method if we want the transmission to be secure.