REST vs SOAP

REST stands for REpresentational State Transfer (REST) which enforces a stateless client server design where web services are treated as resource and can be accessed and identified by there URL unlike SOAP web services which were defined by WSDL.
Web services written by applying REST Architectural concept are called RESTful web services which focus on System resources and how state of Resource should be transferred over http protocol.

SOAP
SOAP, originally defined as Simple Object Access Protocol, is a protocol specification for exchanging structured information in XML form.SOAP is protocol which relies on XML, that defines what is in the message and how to process it,set of encoding rules for data types, representation for procedure calls and responses.


REST vs SOAP

Before going for differences between two lets first see the differences in request header :

Sample SOAP & REST Request message for Weather Web Service :

 

SOAP vs REST Key Differences
Here are some of Key differences between SOAP and REST web Service :

•  REST uses HTTP/HTTPS, SOAP can use almost any transport to send the request(for example we can have SOAP Messages over SMTP),  SOAP is a XML based messaging protocol.
• REST is totally stateless operations where as SOAP supports statefull calls too. 
• SOAP has more strict contract in the form of WSDL between applications, no such contract exists in case of REST.
• AS REST APIs can be consumed using simple GET requests, RESTresponse can be cached, this is not possible with SOAP.
• REST is Lighter, SOAP requires an XML wrapper around every request and response(SOAP Headers, XML tags etc). Thats why REST is preferred choice in mobile devices and PDA's
•  SOAP message format is restricted to XML only where as REST supports other formats too for example JSON.  
• REST is simpler in terms of development as compared to SOAP.
• Browsers can handle REST easily as compared to SOAP as REST is based on HTTP where SOAP is another wrapper over HTTP.

Why to use SOAP ? 
SOAP provides some features like Security, Reliable Messaging, Atomic transaction which are not available in REST API.

• WS-Security  & WS-SecureConversation  Provide support for using security tokens like Kerberos, and X.509.
• WS-ReliableMessaging   Reliable messages delivery between distributed applications in case of failures.
• WS-AtomicTransaction, two-phase commit across distributed transactional resources
• In case where exact specification  of exchange format need to agreed between applications SOAP is better suitable.