SOAP (Simple Object Access Protocol) and REST (Representation State Transfer) are both messaging protocols. Microsoft first defined SOAP in 1998. For a long time SOAP was the standard approach to web service interfaces. However, in the recent years REST has been dominating the playfield.
SOAP vs REST: Key Differences:
SOAP communication protocol uses XML to encode the body of the message and mostly HTTP protocol for its transfer. Microsoft wanted to replace RPC protocol that was limited by firewalls. If your goal is to have robust security, SOAP’s support for WS-Security and WS-AtomicTransactions can come in handy.