REST vs SOAP

• REST: REpresentational State Transfer
• SOAP: Simple Object Access Protocol

Here are some key differences between the 2:

• Underlying protocol: REST uses HTTP/HTTPS, SOAP can use (almost) anything, like HTTP/HTTPS/SMTP, etc
• SOAP uses XML, strict contract in the form of WSDL. REST has no limits and contracts, light weight comparing to SOAP.
• REST is stateless; SOAP supports stateful calls.
• REST response can be cached(GET requests); SOAP cannot.
• REST is easier to develop comparing to SOAP

To summarize, the root cause of SOAP's problems and unpopularity: XML