As an iOS developer, you often need to use a web service for your app. You may need to access remote data party library; parse a social network feed, or even download some assets into your application. This can be possible without using the third as, with the help of various web services.
Web service is basically an application programming interface (API) which is accessed by HTTP. In other language web services are application components which can communicate using open protocols.

Basically is a server that expose methods, you can call these methods with an input and you will receive an output. Web Services are typical web based applications that provide some resources, and these resources can be accessed by any remote system/application over the networks. Web services fulfill a specific task or a set of tasks requested by a client. XML HTTP is the basic web service platform.XML is the language that can be used between different platforms and programming languages.

SOAP (Simple Object Access Protocol), WSDL (Web Services Description Language), REST (Representational state transfer), UDDI (Universal Description, Discovery and Integration) is the elements of the web services. SOAP is an XML-based protocol for accessing a Web Service and let applications exchange information over HTTP.

REST-first of all requests and responses are built around the transfer of representations of resources. A representation of a resource is typically a document that captures the current or intended state of a resource. REST can use HTTP requests using GET (or POST, PUT, DELETE) to call a method of web service and XML or JSON as input and output.

SDWL (Web Services Description Language) is an XML-based language that is used for describing the functionality offered by a Web service. A WSDL description of a web service (also referred to as a WSDL file) provides a machine-readable description of how the service can be called, what parameters it expects, and what data structures it returns.

Return Formats: Two primary web service return formats: XML and JSON. XML is a popular way to structure data using familiar markup language. It is both human and machine readable and is very similar in appearance to HTML. One advantage of consuming data in this format is its structure. Data can be added or removed from the result set.
JavaScript Object Notation (JSON) is a text-based data interchange format derived from the JavaScript scripting language. It is formatted as key-value pairs. SON object is returned from a web service we can convert it directly into native objects such as NSDictionary or NSArray.