Define WSDL,SOAP and REST.

A WSDL is an XML document that describes a web service.

It actually stands for Web Services Description Language.WSDL tells about the functions that you can implement or exposed to the client. For example: add, delete, subtract and so on
SOAP is an XML-based protocol that lets you exchange info over a particular protocol (can be HTTP or SMTP, for example) between applications. It stands for Simple Object Access Protocol and uses XML for its messaging format to relay the information.
REST is an architectural style of networked systems and stands for Representational State Transfer.
It’s not a standard itself, but does use standards such as HTTP, URL, XML, etc